HtH:
+ All normal enemies possible to kill.
- Master is not possible to kill HtH.
Pacifist
+ You can talk Master out of his plans
- Lou doesn't like to talk.
Is it possible to beat the game as pure HtH or pacifist?
Hmm. Depends of what you want out of the game. What constitutes "beating the game" to you.
I made a pacifist character in the sense that he never did the killing himself. I got all the companions, and during combat I just ran around and healed them while they did all the killing. There's a way to get through Mariposa and the Cathedral without initiating combat, so for guys like Lou, I just avoided them completely. Because I had prior knowledge of everything in the game, my pacifist just steered clear of those areas where I knew that combat was unavoidable, and stuck to the places that I could talk my way through. I missed out on some things. But I finished the final objectives, thus I considered it a "beaten" pacifist game.
As for an HtH. Even with Power Armor you can't kill the Master? I'm sorry, I don't think I've ever tried going HtH with the Master.
But there's a way to finish the game without killing (and not even talking to) him, and I think I took that option. Once again, I may not have killed the Master, but I considered it a "beaten" HtH game.
Hope that helps.