PyVISA started as wrapper for the NI-VISA library and therefore you need to
install National Instruments VISA library in your system. This works most of the
time, for most people. But NI-VISA is a proprietary library that only works on
certain systems. That is when PyVISA-py jumps in.

Starting from version 1.6, PyVISA allows to use different backends. These
backends can be dynamically loaded. PyVISA-py is one of such backends. It
implements most of the methods for Message Based communication
(Serial/USB/GPIB/Ethernet) using Python and some well developed, easy to deploy
and cross platform libraries.