I've been slowly converting some of our VM infrastructure over to XenServer from VMWare. Now that XenServer 6.2 is fully open source, it seems to be a good place to transition. Sometimes incredibly simple tasks take you down a rabbit hole you did not plan on going down.
I must confess I spent hours trying different methods and partition schemes. The K.I.S.S. approach is really the way to go here. I found this article to be the simplest and most practical solution, and have updated it here to cover any CentOS 6.4 / XenServer 6.2 issues. It's based on the assumption that you don't have an overly complicated VM partition scheme.