Additional Study Information: The purpose of this study is to: 1. Compare treatment with chemotherapy (paclitaxel) alone to using a combination of nivolumab plus chemotherapy (paclitaxel) treatment in patients with angiosarcoma who have not been treated with paclitaxel chemotherapy alone. 2. Evaluate the effect of nivolumab in combination with cabozantinib on angiosarcoma (cancer) in patients who have either received paclitaxel or similar chemotherapy previously, or whose cancer has grown previously while getting paclitaxel chemotherapy on this study. The chemotherapy drugs, paclitaxel and cabozantinib, work to prevent cells from growing. Nivolumab is a drug that works through your bodys immune system to help the immune system act against cancer cells. Nivolumab has been studied and found to shrink tumors in some people with cancer. Researchers hope to learn if the combination of nivolumab and chemotherapy could shrink your type of tumor, and possibly prevent it from coming back.