First of all, non-blocking I/O has nothing in common with green threads or coroutines, but it can affect how they’re scheduled.
Now:
- Twisted is a classic non-blocking I/O framework — application code is written in async style using callbacks.
- Gevent and eventlet use the greenlet library for coroutines/greenthreads/greenlets. There is one dedicated greenlet for running the eventloop (in case of gevent it’s C-coded libevent’s event loop). When arbitrary greenlet begins to wait for some I/O operation to process, it just gives execution to the event loop, which starts another greenlet for execution (which is ready to do some I/O). This is called cooperative multitasking — each greenlet decides itself when to return control to other greenlets.
- Stackless has tasklets, which are similar to greenlets, but can also be scheduled with a preemptive model — that means the scheduler can stop the tasklet execution at any time and start execution of another tasklet (which is how OS threads and Erlang processes work). Also, Stackless does not provide any non-blocking I/O facilities out of the box, so if you do I/O via stdlib — it will block the entire OS thread, so no other tasklet can execute while you’re waiting on I/O. There have been attempts to provide a port of the gevent library for Stackless but I don’t know how it’s going.