Hardware support for secure virtualization (was: About Xen: maybe a reiterative question but ..)

[Rodrigo V. Raimundo]

With all this discussion some questions went to me:

what's the hardware needed to do full and secure (para)?virtualization ?
is there some arch with this support ever created?
could the virtualization environment be secure if all guest OSes run in
userland? (User-Mode Linux, QEMU without acceleration, ...)

Thanks in advance