I have itchy rash that started on my thighs about 3 to 4 weeks ago, and now it has developed on my face. What could be the cause? Could it be hiv

No. There are characteristic hallmarks of hiv, but "itchy rash" is not one of them. Most common causes would be virus, drug reaction, and allergy to something you came in contact with or ingested. Let a dermatologist evaluate you and tell you for sure.