If you don't have a system that can handle KVM, there's a risk that you'll get related questions on the RHCSA exam that you won't be able to answer, related to the following objectives:
* Access a virtual machine's console.
* Start and stop virtual machines.
* Configure a physical machine to host virtual guests.
* Install Red Hat Enterprise Linux systems as virtual guests.
* Configure systems to launch virtual machines at boot.
Only you can know whether that risk is acceptable to you. (Stef80's point is essentially correct, the tools associated with Xen aren't radically different from KVM -- however, the kernel associated with Xen on RHEL 5 is --ah-- unique.)