Do you know if it's possible to run a second session in a window?

I'm not much hoepful, but... just in case :)

I would like to be able to start a second session in a window of my current one (I mean a second session where I log in as a different user, similar to what happens with the various ctrl+alt+Fx, but starting a graphical session rather than a console one).

Do you know of some software that lets me do it?

Can I somehow run a KVM using my host disk as a the disk for the guest VM (and without breaking stuff)?