In a 4- to 5-quart pan over medium heat, stir onion and garlic in oil until soft, about 8 minutes. Add zucchini and 1 cup chicken broth; simmer until zucchini is tender, about 10 minutes.
Transfer mixture to a blender; whirl until smooth.
Return to pan. Stir in remaining chicken broth, the coconut milk, curry powder, and salt and pepper to taste. Stir until hot.