Please correct me if this question is duplicated. I just came across Unity3D and I just want to ask if it is possible to integrate Unity3D on top of other UIView?
Maybe, but it's far easier to go the other way around. Use Native Toolkit:
http://u3d.as/content/prime31/native-toolkit-unity-to-i-os-bridge/1vj
With this, you can bring up UIViews on top of your Unity3D app. It works great and is very easy to use. This is basically a plug and play version of what's described in the blog posts linked to in Kay's answer, I think.