What type of water pump is also a valve (e.g. will resist head pressure when off)?
I believe the term you are looking for is a positive displacement pump.

However, such a pump is not generally constructed using an impeller and such a pump is only sometimes used for liquids.

Here is a link to a brief discussion of the characteristics of centrifugal pumps versus positive displacement pumps:

http://www.pumpschool.com/intro/pd%20vs%20centrif.pdf

For low pressure water applications a centrifugal pump and check valve in series are usually used to prevent back flow.