How does Christianity view such persons who are Xianpo(s)?

There's a lot of long, well-written answers here, but may I add a short summary?

Christians believe that the souls of deceased people still exist in the afterlife. However, (most) Christians believe that mediums - that is, Xianpo - are either beguiling you through psychological tricks, or are in contact with evil demons who provide them information, but are not actually in contact with the souls of deceased people.

(Also, if you're curious to learn more about Jesus, I'd personally suggest this video, and the Gospel of Luke in the Bible.

In addition to what the others have said, we have Jesus' parable about the Rich Man and Lazarus.

The Rich Man and Lazarus

19 “There was a rich man who was clothed in purple and fine linen and who feasted sumptuously every day. 20 And at his gate was laid a poor man named Lazarus, covered with sores, 21 who desired to be fed with what fell from the rich man's table. Moreover, even the dogs came and licked his sores. 22 The poor man died and was carried by the angels to Abraham's side. The rich man also died and was buried, 23 and in Hades, being in torment, he lifted up his eyes and saw Abraham far off and Lazarus at his side. 24 And he called out, ‘Father Abraham, have mercy on me, and send Lazarus to dip the end of his finger in water and cool my tongue, for I am in anguish in this flame.’ 25 But Abraham said, ‘Child, remember that you in your lifetime received your good things, and Lazarus in like manner bad things; but now he is comforted here, and you are in anguish. 26 And besides all this, between us and you a great chasm has been fixed, in order that those who would pass from here to you may not be able, and none may cross from there to us.’ 27 And he said, ‘Then I beg you, father, to send him to my father's house— 28 for I have five brothers—so that he may warn them, lest they also come into this place of torment.’ 29 But Abraham said, ‘They have Moses and the Prophets; let them hear them.’ 30 And he said, ‘No, father Abraham, but if someone goes to them from the dead, they will repent.’ 31 He said to him, ‘If they do not hear Moses and the Prophets, neither will they be convinced if someone should rise from the dead.’” (Luke 16:19-31)

The impression is given that there are different realms in which people exist that are strictly separated: Earth, Hades, and Abraham's Bosom, for example, with communication and travel between them strictly limited by God.

All have sinned

Regarding the question "are they evil souls?", it's important to realize that the Bible says that all people are sinful--"evil souls" in some sense:

"...for all have sinned and fall short of the glory of God, and all are justified freely by his grace through the redemption that came by Christ Jesus." (Romans 3:23-35 NIV)

and:

10 As it is written:

“There is no one righteous, not even one; 11 there is no one who understands; there is no one who seeks God. 12 All have turned away, they have together become worthless; there is no one who does good, not even one.” 13 “Their throats are open graves; their tongues practice deceit.” “The poison of vipers is on their lips.” 14 “Their mouths are full of cursing and bitterness.” 15 “Their feet are swift to shed blood; 16 ruin and misery mark their ways, 17 and the way of peace they do not know.” 18 “There is no fear of God before their eyes.” (Romans 3:10-18 NIV)

As others have said, spiritism is a specific sin, but all humans are sinners in need of God's grace and forgiveness. This doesn't excuse spirit-ism, but rather it means that we all need forgiveness of our sins through faith in Jesus.

Who Jesus is

Jesus is God's Son, who died on the cross for our sins (he didn't stay dead; God raised him from the dead) so we could be saved from eternal condemnation for our sins through faith in him:

16 For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life. 17 For God did not send his Son into the world to condemn the world, but to save the world through him. 18 Whoever believes in him is not condemned, but whoever does not believe stands condemned already because they have not believed in the name of God’s one and only Son. (John 3:16-18 NIV)

To learn more about who Jesus is, I'd recommend reading the Gospel of John.

There shall not be found among you any one that maketh his son or his daughter to pass through the fire, or that useth divination, or an observer of times, or an enchanter, or a witch, Or a charmer, or a consulter with familiar spirits, or a wizard, or a necromancer.For all that do these things are an abomination unto the LORD: and because of these abominations the LORD thy God doth drive them out from before thee.

Deuteronomy 18: 10-12.

Note Especially : a consulter with familiar spirits ... or a necromancer.

God has stated to Israel that such things as these are an abomination to him.

Christians understand that the dead know nothing (Ecclesiastes 9:5) and the Bible makes it very clear that there is no way we can communicate with them.

The Bible denounces any who enquire of the dead through spirit mediums or necromancers – people who use witchcraft or sorcery to conjure up dead people or try to foretell the future by communicating with them.

Do not turn to mediums or seek out spiritists, for you will be defiled by them. I am the LORD your God. (Leviticus 19:31)

Let no-one be found among you who... practises divination or sorcery, interprets omens, engages in witchcraft, or casts spells, or who is a medium or spiritist, or who consults the dead. Anyone who does these things is detestable to the LORD. (Deuteronomy 18:10-12)

Any person who claims they can speak to the dead and who passes on messages from the dead is under demonic influence.

Whoever makes a practice of sinning is of the devil, for the devil has been sinning from the beginning. The reason the Son of God appeared was to destroy the works of the devil. (1 John 3:8)

Christ Jesus is the light and the life of the world. In him there is no darkness:

Through him all things were made; without him nothing was made that has been made. In him was life, and that life was the light of men. The light shines in the darkness, but the darkness has not understood it. (John 1:3-5)

The powers of darkness and death are controlled by evil forces who oppose everything that is Christ Jesus. Have nothing to do with people who claim to communicate with the dead. Turn, instead, to the light and the life of the world, the resurrected Son of God who is the victor over death and sin.

How does Christianity view such persons who are Xianpo(s)?

What is a Xianpo?

Various types of shamans can be traced to the [Culture of] Hakka [in China]. There are male sorcerers (PY: shunggong) and their female counterpart (PY: shungpo) who cast out devils and other evil influences. Contact with the spirits of the deceased is conducted through mediums (PY: xianpo).

Thus the xianpo are mediums and the Scriptures are very clear that sorcerers and mediums are not allowed to exist in Israel. They are considered an abomination in the eyes of God.

31You must not turn to mediums or spiritists; do not seek them out, or you will be defiled by them. I am the LORD your God. - Leviticus 19:31

You shall not permit a sorceress to live. - Exodus 22:18

10 There shall not be found among you any one that maketh his son or his daughter to pass through the fire, or that useth divination, or an observer of times, or an enchanter, or a witch.n11 Or a charmer, or a consulter with familiar spirits, or a wizard, or a necromancer. 12 For all that do these things are an abomination unto the Lord: and because of these abominations the Lord thy God doth drive them out from before thee. - Deuteronomy 18:10-12

Mediums, magicians, and others who practice occult arts might offer entertainment by means of deception, but some mediums legitimately communicate with either the dead or with demons. The point is that any spirit not from God comes from Satan.

Occultists in the Bible and Throughout History

People have been consulting with the dead for millennia, hoping to find out what is on the other side; believing they can learn the future; desiring control and power.

As the Philistines continued to threaten the kingdom of Israel, Saul sought help from Samuel, but since Samuel was dead, Saul consulted the witch of Endor (1 Samuel 28). As a slave, the power of “fortune-telling” was a source of income to her owners (Acts 16:16-19).

Mediums, magicians, and others who practice occult arts might offer entertainment by means of deception, but some mediums legitimately communicate with either the dead or with demons. If there is any doubt that such a thing is possible, one should remember that “we do not wrestle against flesh and blood, but against the [...] cosmic powers over this present darkness, against the spiritual forces of evil” (Ephesians 6:12).

Whether a medium is speaking to a client’s dead relative or to a demon posing as a deceased loved one, the point is that any spirit not from God comes from Satan.

Why Mediums are Unwelcome in God’s Kingdom

King Saul was already on the way out as the king of Israel, but consulting a medium was “the final step in [his] tragic downfall” because he failed to seek God’s guidance. We are invited to ask God for wisdom. “His way is always best, and He wants to show it to you — because He loves you.”

John Piper designates spiritualism as “the use of psychic or spiritual forces to control another person or the course of events.” People who engage in “divination, soothsaying, augury, sorcery, the use of charms, mediums, wizardry, or necromancy” are doing so in order to “gain […] power over destiny, and for controlling people and events to [their] liking.” “God frustrates” their plans when “directed against His people.” Their plans and the individuals themselves are “abominable in the eyes of God.”

Whether those actions include human sacrifice or merely playing around on the fringes of spiritualism, “every inclination to the deed is evil.” It’s an act of “rebellion” rooted in deception, taking us right back to the garden where the devil tempted Eve by saying “you will not certainly die” (Genesis 3:4).

Piper argues that “we belittle God and exalt ourselves” when we try to discover the unspoken mysteries of our futures and of God’s plans. Jesus is the “Revealer,” appointed by the Father.

Knowing the Difference

Christians need to pay attention because “the coming of the lawless one” will be accompanied by “false signs and wonders” (2 Thessalonians 2:9). Satan and his retinue are “ravenous wolves” wearing “sheep’s clothing” (Matthew 7:15). These demons seek to lead Christians into chaos and pre-Christians to destruction.

They use mediums and astrologers and others of spiritualism to do their work. Prophecy is a "miracle of knowledge, a declaration or description or representation of something future, beyond the power of human sagacity to foresee, discern, or conjecture." Prophecy is a “human report of a divine revelation.”

Christians are warned to “test the spirits” (1 John 4:1). False prophets “reject the word of the Lord” and also say “let us go after other gods” (Deuteronomy 13:2). The gift of prophecy as a revelation directly from God was given “temporarily” in order to “lay the foundation of the early church” and to teach followers before there was a completed bible for reference to God’s “revealed word.” “If someone now claims to have received a ‘special revelation,’ we must test it against Scripture.” Even if it “is consistent with Scripture, then we have to ask why an ‘extra’ word was necessary if its truth is already contained in the Bible.”

The gift of prophecy in today’s church is not a means of satisfying curiosity or easing anxiety about the future, but is understood as “the divine strength or ability to communicate God’s truth and heart in a way that calls people to a right relationship with God.”

Is There Hope for Mediums?

As Piper says, individuals who practice spiritualism are abominable to God, but “not beyond the reach of God’s love.” While preaching in Samaria, “a man named Simon, who had previously practiced magic” and had become arrogant with his fame and skill “believed the good news about the kingdom of God and the name of Jesus Christ” (Acts 8:9, 12).