With the JettyPlugin you are able to run your Cocoon-XML-Application in the servlet-environment. This environment is the main usage scenario of Cocoon. You can use this for previewing or developing.
The Jetty-plugins comes is include in the Hawron-release, so you don't to install it.
Switch to the Servlet-View. If you have a project setup correct you can start the Jetty-Servlet-Container with the 'Start'-button. Wait until the button shows 'stop'.
You can now open your favorite browser and point to 'http://localhost:8888/your/uri'
You can change the port-settings and the session-lifetime.