Chives are the most distinct of the group. Instead of being young onions, they are a separate herb related to onions and garlic. They grow in thin, hollow, grass-like stems with no bulb at all. Their flavor is very mild and fresh, making them ideal as a finishing herb rather than a cooking ingredient. They are best sprinkled raw over dishes like soups, eggs, dips, and potatoes. Garlic chives offer a slight variation with a gentle garlic note.
In short, scallions are your everyday all-purpose option, spring onions bring more depth and texture, and chives provide a delicate finishing touch.