Several years ago I had a conversation with a cool guy working at Opera, Charles McCathieNevile. He told me in mixed English/Italian, "Mobile is tutto casino!" (Mobile is total chaos). This was true in 2004 and it's still just as true today due to the high device fragmentation and difference in screen resolutions.
In order to emulate devices you can use several tools including the ones provided by the major SDK vendors. Usually these tools are time-consuming and sometimes not easy to configure; for this reason, I strongly encourage you to use the web as your development and debugging platform.
One of the greatest advantages to using PhoneGap is that you can use web standards and thus avoid having to continuously test the app in the browser emulator that comes with the SDK of each mobile platform. It doesn't mean that you don't need to emulate the app at all or that you don't have to test it in a real device, but you can safely assume that it's enough to test the...