You do not have to tell your dentist that you have HIV. They may ask you when you join the surgery or on forms, but you do not have to give them this information. Dentists have to follow universal precautions so they do not need to know if you have HIV for safety reasons. They cannot refuse treatment because of your HIV.
It may be helpful to tell them for health reasons. If you are already taking medication and your dentist prescribes you other medication, it is important to know if taking them together will have an unexpected affect. If you do not feel comfortable mentioning it to your dentist, please mention it to your HIV doctor.