The Python interpreter is a program that runs the Python programs you write.
Source code is the statements you write for your program—it consists of text in text files that normally end with a .py extension.
Byte code is the lower-level form of your program after Python compiles it. Python automatically stores byte code in files with a .pyc extension.
The PVM is the Python Virtual Machine—the runtime engine of Python that interprets your compiled byte code.
Psyco, Shed Skin, and frozen binaries are all variations on the execution model. In addition, the alternative implementations of Python named in the next two answers modify the model in some fashion as well—by replacing byte code and VMs, or by adding tools and JITs.
CPython is the standard implementation of the language. Jython and IronPython implement Python programs for use in Java and .NET environments, respectively; they are alternative compilers for Python.
Stackless is an enhanced version of Python aimed at concurrency, and PyPy is a reimplementation of Python targeted at speed. PyPy is also the successor to Psyco, and incorporates the JIT concepts that Psyco pioneered.