Which version of PIP should I use?

  

Must I use pip3 for Python3.x or can I also use pip without any problems?

The version of pip works with any version of Python in theory. Usually Python already comes with a pip version preconfigured for the environment.

  

What are the differences between pip versions?

The same as regular open source software: the features and maturity of the code for bugs.

If you have Python 2x and Python3x, you usually end up having pip and pip3. The difference is that pip installs the libbiotecas in Python 2 and pip3 installs the libbiotecas in Python 3 or its virtualenv.

If you only have Python 3 installed or you are inside a virtualenv, you will only have one pip, and it installs the libbiotecas in Python 3.

To know which pip is pointing to which python installation, you can use:

pip -V