In the United States, a "dining hall" is a room in an institution, like a school, where people sit down at tables to eat meals. It can be formal or informal, plain or fancy, and says nothing at all about how the food gets to the table. The essence of a "cafeteria" is that diner get their food themselves, and do it by sliding trays along rails past various stations where people pass or serve food to them. It is a common way to serve food in institutions, but there are also commercial cafeterias. A "cafeteria" suggests one very long row of serving stations, with people behind it dishing up food as you request it. A "buffet" suggests a big group of steam tables, and diners themselves use ladles or tongs to put the food onto their plates. In the United States, "canteen" is associated mostly with _military_ dining, although it sometimes means a rudimentary snack bar that sells a few items like candy bars.

For me, canteen and cafeteria are facilities you'd find in a workplace or school, where employees or students queue up, choose food and then pay for it. 'Canteen' sounds fairly basic - a place where you eat and then leave. A cafeteria sounds more relaxed - it might be a place where you'd sit around more, chatting to your colleagues or classmates. I'd call the place where Harry Potter and classmates eat a traditional 'dining hall'. If you're eating in a school dining hall, there'd be less or no choice of food. You sit at tables to be served, and no money would change hands.

There is no difference. Although in Australia (where I'm from) a canteen would be a place you buy food, not eat it. Dining hall is slightly (very very very slightly) more formal.

P.S. In the United States, cafeterias are almost universal in schools, hospitals, businesses, etc. and the word "cafeteria" includes both to the serving area and the dining room next to it. And it's often shortened to "caf."