Pxy[u_, a_, 
          b_, c_, d_, ] :=  a*u^3 + b*u^2 + c*u  + d;

Clear[P0];
Clear[P1];
Clear[V0];
Clear[V1];

w  =  
      Solve[
{d  ==  
            P0, 
c == 
            V0, 
3*a + 2*b + c == 
            V1, 
a + b + c + d == 
            P1}, 
{a, b,  c, d}]