The derivative block computes a time based derivative, using a difference ratio (not an analytic derivative).
There is no block that computes the derivative of one signal relative to to another like you are looking for. You can use memory blocks (P(n) -> Memory -> P(n-1)), sums, and division to compute the dP/dV you are looking for.
dP/dV = {P(n)-P(n-1)}/{V(n)-V(n-1)}
The accuracy of this technique depends a great deal on the step sizes of the simulation, and how fast the signals are changing per step. Also, if you are using a variable step solve, consider making the equation discrete (fixed sample time) to avoid uncertainty about the effect of step size.
