The PI is just hardware.
There is no backend.
You need another source with a tuner card for live TV and I think you are saying that you have a MythTV box already set up?
I use OpenELEC (stripped XBMC for PI) as the front end on my PI's and Windows Media Center for the back end on the server.
OpenELEC is available as an image on the PI noob download.
https://www.raspberrypi.org/downloads/