Driving would be too painstaking. Beijing to Xian takes an overnight by train and two days is enough to visit Xian. Beijing to Hohhot takes about 11 hours by train, also two days is enought to visit there. While Xian to Hohhot needs around 22-23 hours, which is really a tough journey. If you can arrive in Xian first, that would be perfect. You can travel from Xian to Beijing, then to Hohhot and return to Beijing at last.