Chives and Green Onions are both entirely different plants so that is the main difference between the Chives and Green onions although they seem similar.
The other difference between Chives and Green Onions is that Green Onions have stalks that are long and slender and end up as small white onion bulbs.
Chives are usually smaller and more tender, typically used raw, and only the green part of the chives are used for cooking or eating.
Chives also do not contain the white bulb at the end of the plant unlike green onions which do.
They also have a slightly different taste in my opinion.