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P-CAV stands for Partial - Constant Angular Velocity. It describes the motion of a body as it rotates at a constant angular velocity. In this case, the CD or DVD disc is being read at an increasing speed up until a certain point, at which the speed will not increase and will hold.
P-CAV then breaks the disk into two zones keeping the speed constant for the inner zone (CAV), but varies the disk rotation for the outer zone (CLV).