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I've been to the park, and during my visit, I didn't see any foxes or coyotes at all. I think it's safe to assume that while it may not be an uncommon experience, it's certainly not an automatic or assured experience.
Canada has a lot of relatively remote and untamed land, and there is a significant amount of wildlife in all of it. Generally, the advice to visitors who do not want to encounter wildlife is to make a lot of noise. This is most important in bear country, since bears are among the most dangerous animals in Canada; noise tends to scare them off.
Typically, if wildlife approaches you, it's because the wildlife is quite tame and used to human interaction. This is usually caused by visitors illegally feeding the wildlife. (This is forbidden in Canadian national parks, but some people do it anyway.) Typically this wildlife won't cause you any harm; it's well aware that it has a good deal getting food from people, and it won't cause you any harm unless you threaten it. If you're concerned, simply being assertive (not aggressive) and making yourself look large is going to have the most effect, but chances are, if a fox approaches you for food and you don't give it any, it will give up and go on to someone or somewhere else.
Once in Alberta on a hike, I had a curious deer follow me for several kilometres. Eventually it bored of us and left. I rather didn't mind it being there. It was clear that it wasn't intending to threaten us. I suspect you'll have a similar experience with the foxes.
If you are at all in doubt or uncomfortable, stop in the park office and speak to a park ranger about the animals. The ranger can give you advice of what areas to avoid or what behaviours you can use to minimize the chances of problems.
Parks Canada has posted some very good advice about animal interactions in Prince Edward Island Park here that you may want to read before you go. There is specific advice about coyotes, which should show natural fear about you.
Have a great time visiting P.E.I.!