My 2 cents: When Red Hat picks a release of kernel, xen, kvm, etc., they tweak, change, and test it until its 'enterprise' ready. If they say you can run a business class server with it, I believe them. Based solely on how well centos works. Even though RHEL 5.4 doesn't have the newest of anything, it does what it does well. Even if I have to put up with fewer features or fixes than the current version, I'm leaving it alone. I'm sure there are guys that can install newer releases of everything needed to run the latest xen, and it might be stable, it might not.