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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.