Conventions are just that. A Convention. Where the primary purpose is for everyone to come together for a specific conference, meeting, or seminar. Usually you can find "hamfest" activities at most (but not all) conventions. But the primary thing is for the convention, the swapfest part is usually an afterthought. Conventions are highly structured events, usually with some kind of formal dinner banquet in the evening time.
Hamfests are specifically for the sole purpose of bringing your unwanted junk to sale, or trade, and for those looking for deals to buy, and usually there are some form of meeting, testing session, and eyeball ragchewing going on, its much more laid back and less structured than a convention.