"What would have happened" is guesswork, since with a different system we'd have had different parties standing in seats, probably battles between candidates from the different wings of the various parties, and very likely a signifi9cantly higher proportion of people actually voting.

However the guesswork consensus is that with AV there would still have been a hung parliament, but probably if a coalition had been set up, it wouldn't have been the one we got landed with. (Though a minority administration, as in Scotland, might have been more probable.)

AV isn't a particularly good system of voting, it's just a lot better than the winner-takes-all system we have which means that two out of three MPs get elected without winning a majority of votes. But it's the only choice we have been given, and you can guarantee that if this referendum is lost there won't be a chance to vote for anything better in our lifetimes or the lifetimes of our children.