The are two ways to integrate Cobertura into Maven.
Put Cobertura into the build section of the pom file, then you have to execute mvn clean cobertura:cobertura to generate the reports. If you have XML and HTML configured, then you get both reports.
Put Cobertura into the reporting section of the pom file, then you have to execute mvn clean site to generate the reports. If you have XML and HTML configured, then you get both reports. Additionally you get a generated site (open target/site/index.html) with all reports integrated e.g. Coberture, Checkstyle, ...