During the kickstart provisioning process you can connect to the virtual framebuffer which is accessible through VNC. It's only available locally so don't try and connect from another machine. From the Xen host you should be able to use:
ssh -X root@YourXenHost.usersys.redhat.com vncviewer localhost:5900
The port may vary according to how many guests you have running. To find out which ports are being used:
# If you are using RHEL5 less than U2 netstat -nlp | grep vnc #otherwise netstat -nlp | grep qemu-dm