Pressure drop of valve is a decrease in pressure as measured between two points in a flowing fluid system. 

Pressure drop that occurs along the direction of flow in a pipe is caused by fluid friction, both in the fluid itself and with the piping surfaces, piping restrictions, or sudden changes in flow path geometry. Pressure drop is directly related to fluid velocity, pecific gravity, viscosity, and the size, shape, and internal roughness of the pipe.

This means that the valve is not solely responsible for pressure drop. Flow rate determines the pressure drop. To certain limits, the higher the flow rate, the greater the pressure drop, and vice versa.