[CentOS] Maybe a dumb question, but....

Folks, I am totally ignorant about xen, and while I've been doing a little reading, I've not yet figured out if xen is running under CentOS or if CentOS is running under xen on this 5.2 version of CentOS. I do know that the whole system from the user standpoint is way slower than it was under 4.7. Heck, I can't even find much on what to do with xen, much less figure out how to use it. Question... is there a kernel for 5.2 that does not have xen built into it, and if so, where is it? Perhaps someone can give me a short tutorial about its value or use, or point me to a place where it's use is described in lay terms? I'm very comfortable with CentOS, but this is a whole 'nother ball game !

Thanks..

Sam