whats the difference between semi hollow body guitars and hollow body guitars besides one being semi and one being hollow body?
One has a block of wood in the middle while the other doesn't. The block of wood allows for less feedback at higher volumes because the top doesn't vibrate (as much). Hope that helps.
then whats the difference between thoses and an archtop guitar
Archtops are solid bodies with another piece of wood added to the top to give a different characteristic to the guitar (like on a Les Paul, it's solid Mahogany, with Maple on top). The arch I don't think does anything special, I think it's a looks thing.
do they all have the same acoustic volume or are some louder unplugged?
the hollow is a little louder but not by much