XBMC v13.1 "Gotham" has been released! See Gotham FAQ for details.
Tvheadend is part of the HTS (Home Theater System) project hosted at http://www.lonelycoder.com/hts/
It functions primarily as a TV streaming backend for HTS's own Showtime Mediaplayer but can be used standalone for other purposes, such as a Digital Video Recorder.
You can access tvheadend's WebGUI (Web User-Interface) by accessing http://ipaddress:9981 and setup your DVB TV-adapter and probe channels. Once all is set, go to XBMC, and Add source "htsp://ipaddress:9982". If tvheadend's WebGUI found any channels, you should now be able to access those and enjoy Live TV function in XBMC.
You can use the webui to schedule recordings, at the moment, you cannot record from within XBMC itself :/
1.2 tvheadend in XBMC
This type of media source will contact to a HTS Tvheadend backend and allows you to watch Live TV.
If you have authentication enabled on your tvheadend backend (which is the default if, say, you've installed it using a .deb package) then you can specify the authentication credentials in the source as follows