I'm Alexey Malashkevich, one of the Pony ORM authors.
For now Pony ORM runs on CPython only because there is no access to frames in other implementations. "Pure Python" means that you can write queries in term of objects using Python generators.
OK, thanks. But isn't it the point of all ORMs? Why the emphasis on pure?
Pure in this case means that you can use Python syntax in order to query a database. This way a database query looks identical to an iteration over a list of Python objects.