AWWA C900 VS AWWA C909

Some differences are described here:

http://www.pweagleinc.com/literature/w/mkt-w-740.pdf

C900 Polyvinyl Chloride Pressure Pipe

C909 Molecularly Oriented Polyvinyl ChloridePressure Pipe

It is my understanding pvco starts its manufacturing life as a smaller, much like normal upvc tube but with about twice its eventual thickness, then that small tube is in effect blown up or stretched larger (balloon-like), under controlled conditions to where that wall thickness is much reduced and the wall material is biaxially affected (arguably like strain hardening) to a higher tensile strength than regular upvc.

You might read the direct comparison ring crushing testing of DIP vs pvc (pages 5 and 6 of http://www.dipra.org/pdf/DIPvsPVC.pdf ) and then DIP vs pvco (pages 7-9 of http://www.dipra.org/pdf/dipVsPVCO.pdf ) and then compare at least the test results concerning short term stiffness of the pvc vs the pvco from both documents. As comparative ring stiffness of pipes is heavily influenced by wall thickness as well as material properties, you will find pvco is considerably less stiff than pvc of the same pressure class due to that reduced wall thickness and similar modulus, and both are in turn CONSIDERABLY less stiff, when even "short-term" values are considered, than ductile iron pipe.
The long-term stiffness of plastic pipes (slower than 1/2" on diameter per minute, load application, not reported there) is incidentally, typically just a fraction of the value obtained for plastic pipes in the short-term, parallel plate loading tests.

The significance of this is summarized by DIPRA in both these publications, ""Because pvc(o) pipe has far less stiffness than Ductile Iron, the importance of soil stiffness is greater for pvc(o). This means that with pvc(o) pipe, bedding conditions and on-the-job installation inspection are much more critical."