Howtoforge posted a tutorial about setting up Xen on CentOS 6.3

This tutorial provides step-by-step instructions on how to install Xen (version 4.1.x) on a CentOS 6.3 (x86_64) system. Xen lets you create guest operating systems (*nix operating systems like Linux and FreeBSD), so called "virtual machines" or domUs, under a host operating system (dom0). Using Xen you can separate your applications into different virtual machines that are totally independent from each other.

  Virtualization With Xen On CentOS 6.3 (x86_64) (Paravirtualization Hardware Virtualization)