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run "Xvnc :0" on DomU, this will start an X server that uses a pseudo-frame buffer as video-hardware and serves that framebuffer via vnc >protocol.
>now run "gnome-session" on DomU to start the whole gnome desktop >infraestructure on the guest OS (as gdm will do)
 
>Now, from dom0 that is the only one with a physical screen attached at, >launch vncviewer and connect to domU-ip-address:0, this will show the >remote virtual desktop.