There is a white paper regarding virtualisation with recommendations about the setup. Unfortunately this white paper is on the EA site so you will only be able to get it if you are an EA customer. Unless the vCampus guys negiotiate moving it to the vCampus library, over to you Amhad.
The whitepaper Michael refers to is available here:
Does OSIsoft support the PI System running on virtual machines (such as VMWare or Microsoft Virtual Server)?
Note that PI Server has not been tested on Citrix Xen Server. There have only been a couple requests regarding this. That said, most customers have been successful with using various virtualization technology as long as they follow the recommendations outlined in this article.