Hmm...sometimes I think you guys try to think up hard questions for we native speakers to answer :-D
I don't thinkn I would ever say "This is a TV" because everyone know what a TV is. Maybe if I was introducing the TV to someone from, say, a remote village in Africa who had never seen one before, I might use "This is a TV".
Similarly, I wouldn't say "It's a TV" unless someone nearby wasn't sure what an object (which happened to be a TV) was. Perhaps a friend of mine and I are watching a music video or cartoon and they show a TV on that show or video that's maybe shaped oddly; I think it's a TV but I'm not sure, so I ask my friend "What is that thing"? "It's a TV."
I really don't think you'll ever need either of those sentences, fisher. Substitute TV for something else...maybe a new technology of some sort. Then you'll need them.