I'm not that familiar with xmlrpclib, but as far as I know from a quick look xmlrpclib lets two programs, possibly running on different computers, communicate over HTTP.
Pykka helps multiple threads (or eventlets if using pykka.gevent) in a single program to communicate in a safe and easy matter without requiring the developer to manage locks and whatnot that he would have if he used plain threads. Under the hood Pykka is just an abstraction on top of threads and queues.
It's not uncommon for actor frameworks, like Akka, to support communication between actors on different computers, but there is currently no support for remote actors in Pykka.