Is your intent only hour building or attend a flight training course?
As far as I know, you do not need any specific visa to build up hours.
I don't know where you are from, but I am Italian and if I were to go to USA with the only purpose of building up hours my electronic passport would be enough to enter the country and fly hours. I would enter the country as a tourist and not as a student. Building up hours doesn't mean that you are enrolled in a course.