This isn't really a howto per se but rather a pointer to RPMs for KVM on CentOS/RHEL.

You can get KVM 80 for x86_64 here.

http://www.lfarkas.org/linux/packages/centos/5/x86_64/

The same site doesn't seem to have i386 RPMs but they do have i386 SRPMS that you could build.

http://www.lfarkas.org/linux/packages/centos/5/SRPMS/

I don't have time to do any sort of tutorial on building KVM SRPMs because I'm not currently using KVM for any projects. In the future I'll get to it but probably not too soon.