Hardstyle is a variation of hard trance, with a slightly techno edge to it. Still has huge hook lines though, like trance. And plenty of cheesy movie quotes...
"proper" techno is fairly minimal, doesn't have much reliance on melody, is very rigid in structure and is all about the 'groove'.
Simply (and this is an oversimplification) techno is for nightclubs and hardstyle is for dayclubs (those dirty little clubs that open when the nightclubs close in the morning..)