Not to sound like a noob but Xen really needs a good GUI of its own. Xen seems to have as many or maybe more features than some of the best VM tools out there. To me, one of the only compelling reasons to support a GUI is to give a more user friendly way of using most (or all) features a program has to offer in an organized manner. Certain programs eventually become so big that their manuals are just annoying to filter through and daunting for new users. A properly made GUI can make enough of a difference that users can understand the significance of whether something is needed or not, if they're using something properly, or understand what something does without looking at a help file.