Gavin's notes on getting gitian builds up and running using KVM:

These instructions distilled from:
  https://help.ubuntu.com/community/KVM/Installation
... see there for complete details.

You need the right hardware: you need a 64-bit-capable CPU with hardware virtualization support (Intel VT-x or AMD-V). Not all modern CPUs support hardware virtualization.

You probably need to enable hardware virtualization in your machine's BIOS.

You need to be running a recent version of 64-bit-Ubuntu, and you need to install several prerequisites:
  sudo apt-get install ruby apache2 git apt-cacher-ng python-vm-builder qemu-kvm

Sanity checks:
  sudo service apt-cacher-ng status   # Should return apt-cacher-ng is running
  ls -l /dev/kvm   # Should show a /dev/kvm device


Once you've got the right hardware and software:

    git clone git://github.com/bitcoin/bitcoin.git
    git clone git://github.com/devrandom/gitian-builder.git
    mkdir gitian-builder/inputs
    cd gitian-builder/inputs
    # Inputs for Linux and Win32:
    wget 'http://miniupnp.tuxfamily.org/files/download.php?file=miniupnpc-1.6.tar.gz'
    wget 'http://fukuchi.org/works/qrencode/qrencode-3.2.0.tar.bz2'
    # Inputs for Win32: (Linux has packages for these)
    wget 'https://downloads.sourceforge.net/project/boost/boost/1.50.0/boost_1_50_0.tar.bz2'
    wget 'http://www.openssl.org/source/openssl-1.0.1b.tar.gz'
    wget 'http://download.oracle.com/berkeley-db/db-4.8.30.NC.tar.gz'
    wget 'https://downloads.sourceforge.net/project/libpng/zlib/1.2.6/zlib-1.2.6.tar.gz'
    wget 'https://downloads.sourceforge.net/project/libpng/libpng15/older-releases/1.5.9/libpng-1.5.9.tar.gz'
    wget 'ftp://ftp.trolltech.com/qt/source/qt-everywhere-opensource-src-4.7.4.tar.gz'
    cd ../..

    cd gitian-builder
    bin/make-base-vm --arch i386
    bin/make-base-vm --arch amd64
    cd ..

    # Build Linux release:
    cd bitcoin
    git pull
    cd ../gitian-builder
    git pull
    ./bin/gbuild --commit bitcoin=HEAD ../bitcoin/contrib/gitian-descriptors/gitian.yml

    # Build Win32 dependencies:
    ./bin/gbuild --commit bitcoin=HEAD ../bitcoin/contrib/gitian-descriptors/boost-win32.yml
    ./bin/gbuild --commit bitcoin=HEAD ../bitcoin/contrib/gitian-descriptors/deps-win32.yml
    ./bin/gbuild --commit bitcoin=HEAD ../bitcoin/contrib/gitian-descriptors/qt-win32.yml

    # Build Win32 release:
    ./bin/gbuild --commit bitcoin=HEAD ../bitcoin/contrib/gitian-descriptors/gitian-win32.yml

---------------------

gitian-builder now also supports building using LXC. See
  https://help.ubuntu.com/12.04/serverguide/lxc.html
... for how to get LXC up and running under Ubuntu.

If your main machine is a 64-bit Mac or PC with a few gigabytes of memory
and at least 10 gigabytes of free disk space, you can gitian-build using
LXC running inside a virtual machine.

Here's a description of Gavin's setup on OSX 10.6:

1. Download and install VirtualBox from https://www.virtualbox.org/

2. Download the 64-bit Ubuntu Desktop 12.04 LTS .iso CD image from
  http://www.ubuntu.com/

3. Run VirtualBox and create a new virtual machine, using the
  Ubuntu .iso (see the VirtualBox documentation for details).
  Create it with at least 2 gigabytes of memory and a disk
  that is at least 20 gigabytes big.

4. Inside the running Ubuntu desktop, install:
  sudo apt-get install debootstrap lxc ruby apache2 git apt-cacher-ng python-vm-builder

5. Still inside Ubuntu, tell gitian-builder to use LXC, then follow the "Once you've got the right
  hardware and software" instructions above:
  export USE_LXC=1
  git clone git://github.com/bitcoin/bitcoin.git
  ... etc