Six kinds. Suppose that two kinds of story books are story one and story two, and three kinds of science and technology books are science and technology one, science and technology two and science and technology three, then all possibilities are: story one, science and technology one; Story 1, technology 2; Story 1, technology 3; Story 2, Technology1; Story 2, technology 2; Story 2, technology 3.

Xiaohua can choose a story book first. There are two possibilities:

1. Choose a story 1. After that, Xiaohua needs to choose one of the three science and technology books, namely: story 1, science and technology1; Story 1, technology 2; Story 1, technology 3.

2. Choose Story 2, and then Xiaohua needs to choose one of the three science and technology books, namely: Story 2, Science and Technology1; Story 2, technology 2; Story 2, technology 3.