Do compression stockings prevent dvt? so if you wear them does that mean you won't get dvt?

Nope: They do prevent DVT. However a thrombus/clot is a result of multiple factors, and a compression stocking can control (that too a degree) - one of them. There fore it cannot guarantee "that you won't get a dvt". But they do CUT down the risk of getting a DVT - so if your situation appropriates it, please do use it. I definitely ask my parents to use it while flying for long times.

Dvt: They are the best possible choice yet dvt can sometimes occur