$ pyenv versions
* system (set by /Users/selochanlee/.pyenv/version)
install specific version
# check the installable version list
$ pyenv install --list
...
# install specific version
$ pyenv install 3.6.1
...
# confirm it whether installed or not
$ pyenv versions
...
create virtualenv with pyen and activate it
$ pyenv virtualenv 2.7.13 <venv_name>
...
# activate the installed version
$ pyenv activate <venv_name>
...