Parabellum Knall is what I've read in some places. Other places I've read said it stands for Pistole Automatische Knall, and yet others claim it is Pistole Automatik Knall. Is that what you were asking about?
As far as RIVO is concerned, I'm not sure and I can't find anything on it. It seems the biggest manufacturer of blanks is RWS/Umarex, Walther, and Fiocchi. Maybe someone else has heard of this headstamp.