1. Install Vagrant.
2. cd into the folder where you checked out ycmd.
3. $ vagrant up && vagrant ssh. This will take a while because the VM is being built and set up. Only needs to happen once though.
4. You are now in the VM. Run the tests with $ ./run_tests.sh.
5. Hack away. When done, exit the ssh connection with exit.
6. $ vagrant suspend so that you can quickly get back to hacking later.
7. Later on: $ vagrant resume && vagrant ssh. This will be much faster.

That's it!

You can switch between Python versions with pyenv global 2.6.6 and pyenv global 3.3.0.

If you ever feel like you've screwed up the VM, just kill it with vagrant destroy and then run vagrant up again to get to a clean state.