It can be done just by using the --python. When you are creating your virtualenv instance to specify the Python executable file you should use the following command:-
e.g:-
virtualenv --python=/usr/bin/python2.6 <path/to/new/virtualenv/>
For Python 3.6 you can use the following command for creating the virtual environment:
python3 -m venv <myenvname>
For the python version (3.3+), use either the above method or the script pyvenv the command is as follows:-
pyvenv /path/to/new/virtual/environment