One of possible solutions is to use Emma. You shall setup code instrumentation in your source code modules by using instrument goal:
http://mojo.codehaus.org/emma-maven-plugin/instrument-mojo.html
After successful compilation and instrumentation, tests execution will generate coverage data. Then You can execute emma standalone tool to generate report based on it:
java emma report -r txt,xml,html -in coverageA.em,coverageB.em,coverageC.em,coverage.ec -sp srcA/,srcB,srcC
coverage*.em shall be replaced with proper paths to generated by Emma metadata in source code modules, coverage.ec is path to coverage file generated in test module, src* directories shall be replaced with paths to source code directories. Here is detailed documentation:
http://emma.sourceforge.net/reference/ch02s04s03.html
You can do it also with jacoco (also in a quite tricky way) but due too low reputation I cannot put more than 2 links. So like it like it! :)