Clonezilla would be the easiest in terms of making it into an image of the hard drive and all that.
I'm not the most knowledgeable in Xen, however simply for ease of use I'd suggest VirtualBox since it's designed around that.
Worse case scenario though, you can always use
Portable VirtualBox. It's not perfect, but meh.
Edit: Oh geeze you said for Virtual Environments. Yeah my bad, apparently I'm not awake enough. I was more thinking in terms of having a "portable" version running around.