Despite the different names, scallions and green onions can be used interchangeably. Each plant is harvested early from the Allium cepa species of onions.
Chives aren't a form of an onion — they're part of the Amaryllidaceae family of flowering plants. They are thinner than scallions and green onions and lack a bulb at the bottom.
Chives have a much more mild onion taste, with a bit of a garlicky bend. This makes them easier to eat raw and slice thinly to use as a garnish on almost anything.