What is the difference bitween Bitmap and Pixmap?

The term "pixmap" is short for "pixel map. A pixmap stores and displays a graphical image as a rectangular array of pixel color values.
while a pixmap that uses only a single bit to denote the color of each pixel is often referred to as a bitmap. Bitmap is also sometimes used to refer to any pixmap.

Reference

Look at here to read more about PixMap.

File extensions

bitmap is .BMP

pixmaps is .XPM

There are monochrome bitmaps and color bitmaps.

Bitmap is a synonym to a monochrome bitmap.

Pixmap is a synonym to a color bitmap.

Graymap is a synonym to a color bitmap.

Monochrome bitmaps use 1 bit to store a color, hence max 2 colors.

Color bitmaps use several bits for storing a color. The more bits per pixel it dedicates, the more colors this pixmap / graymap / color bitmap will support.