Hi all,
I would like to report a possible bug that would affect all versions of WFS, on all versions of GeoServer I have tested so far.
Let's take WFS 2.0.0. It is my understanding that the KVP requests are the translation of the default POST requests in XML in a way that can be used with a simple GET request. In particular, a feature name is defined as a QName (XML qualified name), which implies that XML prefixes should be defined, as they would be in a standard POST request. This is the specific purpose of the NAMESPACE parameter, as you can see in the OGC standard (09-025r1 http://portal.opengeospatial.org/files/?artifact_id=39967) in §6.3 and §7.6.6.
In my experience, no version of GeoServer that I have seen seems to be able to understand this parameter. The following URLs reference one of our tests server, please be gentle with it 
We have a feature named Isogeo:OpenBeerMap_IDF, Isogeo being the prefix by being related to the namespace ... isogeo (sorry for that).
If you call with the default prefix, you get an answer when you should not as the prefix has not been declared: http://noisy.hq.isogeo.fr:6090/geoserver/wfs?service=WFS&version=2.0.0&request=GetFeature&maxFeatures=1000&typeNames=Isogeo%3AOpenBeerMap_IDF
If you declare the prefix, it works as expected: http://noisy.hq.isogeo.fr:6090/geoserver/wfs?service=WFS&version=2.0.0&request=GetFeature&maxFeatures=1000&typeNames=Isogeo%3AOpenBeerMap_IDF&namespaces=xmlns(Isogeo,isogeo)
If you declare another prefix, it does not work when I think it should : http://noisy.hq.isogeo.fr:6090/geoserver/wfs?service=WFS&version=2.0.0&request=GetFeature&maxFeatures=1000&typeNames=foo%3AOpenBeerMap_IDF&namespaces=xmlns(foo,isogeo)
What do you think?
Mathieu Cartoixa