There is no difference. Officially, only ADHD
exists. It comes in three varieties: ADHD
predominantly hyperactive, ADHD predominantly inattentive, and ADHD mixed type. In other words, you don't need to be hyperactive in order to be diagnosed as ADHD.
The term ADD is merely a popular abbreviation frequently (mis)used to indicate that someone is inattentive and does not display symptoms of hyperactivity.