Beyound the SSH X11Forwading way I succeded in XDMCP!
Use Xnest or Xephyr in Domain0:
Xnest :1 -geometry 1280x800 -query 10.0.1.x
Xephyr :1 -screen 1280x1024 -query 192.168.1.x
The only thing you have to do is to enable remote login in your VM or DomainU by manually editing /etc/gdm/custom.cfg or using the gdmsetup GUI program. Here CentOS 5 DomainU is taken as an example. In Ubuntu things will be a little differrent.
This approach is very different to VNC which is like Microsoft's RDP (screen capture) while XDMCP utilizes your Domain0's graphic power to assist DomainU system. That is comparable to Xen VGA passthrough in terms of performance.
Remember X11 was specifically designed to be used over network connections rather than on an integral or attached display device. X features network transparency: the machine where an application (the client application, for instance Firefox in your VM or DomainU) runs can differ from the user's local machine (the display server, that is the X11 in your Domain0). This approach allows both 2D and 3D operations to be fully accelerated on the user's local X server.