LXC - lightweight virtual machines

The best tool to create virtual machines that I've ever used is lxc
I use it to isolate environments in my machine and in CI too

in ubuntu,

sudo apt-get install lxc
sudo lxc-create -t ubuntu -n <name>
sudo lxc-start -n <name> #username and pass is 'ubuntu'

that's it!
You have access to the system file via /var/lib/lxc/<name>

http://lxc.teegra.net/