Khosrow Parviz is their rendition of the name of the last great king of the Sassanian dynasty, Chosroes II Parvez (590-627), best known perhaps for his war with the Byzantine Empire during the reign of Heraclius (Chosroes captured the True Cross but Heraclius later regained it).
According to Richard Frye, qasr is an Arabic term for the fortified inner residence of a ruler.