Hmmm- I always thought green onions and chives were the same thing. I just chop up the green portion and throw away the onion bulb for recipes that call for chives. Yikes!
Chives are smaller -- the diameter of coffee stir sticks (or even narrower, like grass), and usually sold with the herbs in those plastic clamshells, cut about 5" long. They have a bit milder flavor than bigger green onions, and the way they're sold in the store they have the bulbs already cut off.
Sometimes you can find wild ones growing on your lawn. If you pull them yourself, you'll see the bulbs on the bottom are way smaller than what you see with green onions--nothing you would ever use.
Now, the difference between green onions and scallions -- that I'm not sure of.