For Xen, GRUB, the default boot loader for is required1
For KVM, the system must have a CPU with virtualization support.
Sufficient storage space for the guest operating systems. A minimal command-line Fedora system requires around 600MB of storage, a standard desktop Fedora system requires around 3GB.
Generally speaking, at least 256 megs of RAM per guest plus 256 for the base OS. Practically speaking, it is hard to do work with virtualization with less than 1 GB of RAM.