From MythTV Official Wiki
Xbox Media Center as described by xbmc.org:
- "XBMC media center is a free cross-platform media-player jukebox and entertainment hub. XBMC is open source (GPL) software available for Linux, Mac OS X, Microsoft Windows operating-system, and the Xbox game-console. XBMC can play a very complete spectrum of of multimedia formats, and featuring playlist, audio visualizations, slideshow, and weather forecast functions, together with a multitude of third-party plugins."
Mythtv plugin for XBMC can be found Plugin for XBMC is here.
XBMC as a MythTV frontend
XBMC can be used as a MythTV frontend in several different ways, all of which have been outlined on the Xbox Frontend page.