Simmer milk with rice, sugar, cardamom, and saffron in 7- to 8-quart heavy pot, stirring often, until reduced by half, 45 to 50 minutes. Discard cardamom.
Heat ghee in a heavy skillet over moderate heat until melted, then cook pistachios and raisins, stirring, until nuts are lightly browned, about 2 minutes. Stir half of pistachio mixture into payasam and sprinkle remainder on top.
