ContourPlot[p V,{V,0,1},{p,0,1}]

S[p_,V_]:={T=p V; En=3/2 T;3/2 Log[En]+Log[V]}

ContourPlot[S[p, V],{V,0,1},{p,0,1}]

p1=ContourPlot[S[p, V],{V,0,1},{p,0,1},ContourShading->None,ContourStyle->Red,Contours->20]

p2=ContourPlot[p V,{V,0,1},{p,0,1},ContourShading->None,ContourStyle->Blue,Contours->20]

Show[p1,p2]