I you want to talk about how they got to power, well, fascists usually got there by votes, and communists by revolution. It's rare--but not unheard of--for a communist gov't to be voted into power.

But the end results end up being remarkable similar; they're both dictatorial, and both use state-sponsored violence to suppress or eliminate competing political ideology. (I want to be clear that I'm not talking about Marxism specifically; Marx was opposed to the existence of a state. Lenin, Stalin, Mao, Castro, et al. were authoritarian.) As long as you are part of the political in group, you're golden. Once you're out, you're an enemy of the state.