Para-virtualisation works on all x86 CPU's made in the last years. For full-virtualisation, hardware support is required.
(on the Xen mailinglist on 15 februari 2007, Mats Petersson from AMD)
The “pacifica” which is now caled AMD-V is fairly easy to determine - if it uses Socket F, Socket AM2 then it's AMD-V compatible, unless it's a Sempron processor. Another key would be to look at the memory type, as all DDR2 processors (excluding Sempron's again) are AMD-V capable.
Intels products are specified on http://processorfinder.intel.com
- once there, you select your processor group through the selection box, then pick a processor model and you'll find a list of “supported features” which will list the “Virtualization technology” if the processor supports it, and not if the processor doesn't. Of course, you'll also have to make sure the motherboard in question is supporting VT enable/disable, or you may end up finding that VT is automatically disabled…