Very helpful. thanks!
Now I hope one step further in virtual X to run windows manager from a different partition.
I have two partition one for booting system, the other for new version ubuntu. I need new version gnome/kde session can be show at booting system's virtual X.
Because X must be started by root, startkde need to be started by normal user. I have tried this.
sudo chroot /media/sda3 su username -c "/usr/bin/startkde"
It works but when switching back and forth again, system stop there no response.