That sounds similar to my setup, except that my raid-5 is hardware and pretends to be a scsi device. Which version of xen are you using? I'm on xen-3.0.2-0.FC5.3, and using kernel kernel-xen0-2.6.16-1.2122_FC5 and kernel-xenU-2.6.16-1.2122_FC5.
I'm using the Xen 3.0.2 hypervisor and xen-kernel from XenSource download page, installed on a dom0 Red Hat's Enterprise Linux ES 4.1 Update 3.