This has nothing to do with kernel-xen ... You'll have the same problem with the standard kernel. Upstream did compile kernels for el5 without CONFIG_FB_VESA ... so no framebuffer at the console.
See the report on the upstream bugzilla system : https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=236195
Very useful info Fabian. I'll wait the next release update with impatience.