If you don't want to record anything and just want to watch tv, mythtv is overkill. You're looking for something like kdetv, tv-viewer, xawtv or tvtime.
If you do want the whole PVR experience (and hey, why wouldn't you?
), you'll need to install both the backend and the frontend. The backend does the scheduling and recording (and operating of the TV tuner card), the frontend is the "viewer". LiveTV in MythTV is just a current recording that you're watching in real time.
Other PVRs are Freevo, XBMC, Tvheadend, and boxee.