You can consider *on tv* a fixed phrase.  Something is *on tv* if it's playing on a channel somewhere, and you can tune into it.  In this case it would imply watching it through broadcast or cable TV as opposed to playing a DVD.

*X through Y* means in some sense, you have to "pass through Y" to get to X, or that Y is some type of "gateway" to X.  

So the meaning is "Y will enable you to do X if you use Y."  If Y is a person, it means you have to talk to him/her to get or see X.

It's very well known that you can use a TV to watch things, so *watch a film through TV* sounds very awkward like you are stating something very obvious.

If the X in "You can watch X through TV" was something you don't normally use a TV to see, then it would make sense.  For example:

> I have a security camera.  You can watch the outside through the TV.