- You can watch a film through tv.
- You can watch a film on tv.
What is the difference between those prepositions?
In virtually every language, prepositional phrases are the most difficult parts of that language for foreign speakers to learn. Native speakers grow up using idiomatic, prepositional phrases in their native language and accept them as natural and normal.
Sometimes, there is only one preposition that can be used. E.g., my cat is in physical contact with the floor, and is entirely supported by the floor, so my cat is 'on' the floor. There is no other preposition I can use that would meet those conditions.
Sometimes, the selection of a preposition is more arbitrary and difficult to explain. E.g., My brother is on holiday. But, my brother is not in physical contact with 'a holiday', nor is he entirely supported by it, so why do we use the preposition 'on'. No particular reason that I know of, it's just what we say. In French, this would be translated to, 'Mon frère est en vacances', which literally translates to, 'My brother is in holiday'. It sounds odd in English, but perfectly normal in French.
Moving on to your question, the correct way to say this in English is:
You can watch a film on TV. (Note: TV is uppercase.)
Why? Because that is the idiom we use in English. We could have said:
You can watch a film in TV.
You can watch a film through TV.
You can watch a film over TV.
You can watch a film with TV.
Any of these prepositions could have been adopted in English for using a TV to watch movies, but for some reason we selected 'ON' instead. If you use any other prepositional phrase than 'on TV' people will probably know what you mean, but they will think that your English is non-standard.
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.
Yep, I goofed. There is a difference between on tv and on the tv.
On TV is short for "available on a channel broadcasted by a TV station where you can watch it using a TV if you tune in to the right channel."
On the TV means "on a physical TV set." It can mean the same as the above but only rarely and it comes off as old-fashioned (more than broadcast TV is already).
Here's an updated example that takes these nuances into account:
There are nature shows on PBS. Tune into them, and you can experience nature through TV.