They are almost the same, except cultural diffusion is the spread of new ideas, religions and etc. Diffusion spreads but you don't know what you mostly talk about.
Cultural diversity is the variety of human society and culture in a specific region or even in the world. It exists between people, such as language, dress, tradition and the way people interact with...
What do you think? I'm sure your teacher or whoever is asking this question would really appreciate it and think more highly of you if it came from your own brain!