There's a level between virtualenv and LXC, and that's schroot. Combine it with a CoW filesystem and that will cover everything you mention. Although personally

There's a level between virtualenv and LXC, and that's schroot. Combine it with a CoW filesystem and that will cover everything you mention. Although personally, I find LXC very lightweight. Note that in my article, I did point out that I do still use virtualenv sometimes :)