Usually generated sources should be created in the target dir. But how do I handle classes that are only used for test? I dont want that these classes get packaged in my jar
Use maven build helper plugin's add-test-source goal to add your generated test source files to the build -> http://mojo.codehaus.org/build-helper-maven-plugin/add-test-source-mojo.html
It ensures that the directories added by this goal will be picked up automatically by the compiler plugin during test-compile phase of the build.
EDIT
Here is the example of how to generate code for testign with cxf-codegen-plugin