You have a couple of options.
1. Pay for RHEL AS.
2. Go with CentOS, which is a community rebuild of RHEL, and comes in the AS version by default. It does not have some of the value-add stuff that might be useful; this might include some of the virtualization management bits you are looking for.
3. Use VMware Server instead of Xen. VMware Server is free.
I'm currently doing VMware Server 1.0.x on CentOS 4, and it meets my needs.