Unfortunately, 14.04 shipped with a broken pyvenv. According to this launchpad thread the issue will be resolved in the upcoming 14.04-1
Using this method you can install a Pyvenv environment without pip and then manually install pip after the fact.
pyvenv-3.4 --without-pip myvenv
source ./myvenv/bin/activate
wget https://pypi.python.org/packages/source/s/setuptools/setuptools-3.4.4.tar.gz
tar -vzxf setuptools-3.4.4.tar.gz
cd setuptools-3.4.4
python setup.py install
cd ..
wget https://pypi.python.org/packages/source/p/pip/pip-1.5.6.tar.gz
tar -vzxf pip-1.5.6.tar.gz
cd pip-1.5.6
python setup.py install
cd ..
deactivate
source ./myvenv/bin/activate
Source: http://askubuntu.com/questions/488529/pyvenv-3-4-error-returned-non-zero-exit-status-1