Besides, Foro means forum. I envision XenForo becoming more than just forum. So, Foro slated to be outdated.
Xen is a catchy name and easy to remember. Competitors have short or abbreviated names: vB, IPB, PHPBB, MyBB. I think Xen would fit nicely in the line.
XenForo is in its infancy and name can be changed without harm right now.
What do you think?