Difference among Comet servers and XMPP servers

In my next planned project I suppose to implement an online classroom website in which I want to share a black board (Say a simple text area) among the Tutor and the participating online students {These all are logined through the website}. Whatever the text Tutor writes over the Black board has to be seen by the other participants in Real-time.

I want to use Java as my development platform.

To implement this I started googling on PUSH technology. I read that XMPP implementation servers and XMPP libraries can be used to implement near-realtime applications like collaberative applications/mutiparty games/IM applications etc.,
But I also read about BlazeDS usage for real-time and low-lattency web applications.

My questions are,

1. what is the difference among these XMPP java based implementation and BlazeDS? Is not both the techs final goal is to achieve low-lattency web apps using PUSH technology?




2. What is the difference among Comet servers and XMPP Servers? Do they just differ in the way they implement the PUSH technology or any thing else?

I am confused. Please someone explain me these little more so that I will know what I do before I do the things and where to start on next in my project.

Thanks

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