Spring onions are a slightly more mature version of the same plant. The key difference is that they develop a small, visible bulb at the base. This gives them a stronger, sweeter onion flavor closer to regular onions but still milder overall. They are excellent for roasting, grilling, or sautéing whole, as both the bulb and greens are edible and flavorful. Spring onions are especially popular in Mediterranean and Asian cooking where a fuller onion taste is desired without the intensity of mature onions.
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.