Qemu fine for simple no admin installs and pendrive and low memory maybe.
Virtuabox and VirtualPC are easy for beginners and really work well.
Vmplayer is easy if you have prebuilt vm's and or don't mind some text edit or use web site that has page to build vmx.
I don't care for either server apps.
Xen can't be used
Almost all virtual machines at vmware are free that are normally free. All should state it on download page. Normally MS and demos would say it. Fedora is free.