Work done is zero because in a cyclic process 
Anticlockwise work done and clockwise work done will have opposite signs. Since it's clear in the graph that Triangle AOD is equal to BOC. The work done is equal n opposite n hence cancels out each other. So net work done is zero
						

							
HELLO THERE!
 
From the Graph, it is clear that the system is first expanding (A to B).
Then,the system is compressing (B to C)
Next, the system is again expanding (C to D)
After that, the system is compressing (D to A).
 
Note: The paths in which volume decreases, the system is compressed, and the paths in which volume increases, the system is expanded.
 
Now as you can see, the clockwise rotation and the anticlockwise rotation is equal, and the expansion of the system is equal (in extent) to the compression, so the total (net) work done becomes ZERO, though area under PV curve is not zero.
From the cyclic process,we can conclude one more thing: that, the total Internal energy change is also ZERO.
 
Thanks!!