Maven 3 changes some things in the way the reporting works (which is something the kernel maven pom file supports). From the maven docs.
A major aim of the refactoring in Maven 3 was to decouple the Maven core from Doxia and to allow arbitrary reporting systems to be developed. For this reason, all reporting related code has been removed from the core of Maven 3 (MNG-4162).
As a result, the 2.x versions of the Maven Site Plugin will not work with Maven 3. Using such versions of Maven Site Plugin with Maven 3 won't generate any reports from reporting plugins, only hand-written Doxia documents (apt, xdoc, ...) will be rendered.
You need to use version 3.x of the Site Plugin, in order for it to work with Maven 3.