In HCP, the [2D] Hexagonal packed layers are arranged in ABAB form. The same happens in Primitive Hexagonal Unit Cell. So does that mean they are the same? If not what differentiates one from the other?
the same happens in primitive hexagonal unit cell
Herein lies the gap in your concept: The same does not happen in primitive hexagonal unit cell. Here's an image to clear things up
The left one is a primitive hexagonal unit cell whereas the right one is a HCP lattice. To use your terminology, a simple primitive cell has packing of the type AAAA.. whereas a HCP cell has packing of the type ABAB..
I highly recommend reading your textbook or referring to some more material on solid state chemistry, such as this SlideShare presentation on basic crystallography.