[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