I wonder if this feature already available if not how to approach it.
I need to generate either xml file or Rss feed with list of exhibits created.
the xml file should include the title of the exhibit, description, tags associated with and slug.
any idea is appreciated.
Exhibit Builder doesn't currently have a feature like this.
It's a good idea, and several people have suggested or asked for it before. It's among the (many) things we're considering for the Exhibit Builder in Omeka 2.0.
For the time being, your best bet would be to write a plugin that reads the Exhibit data and creates the XML or RSS output you're looking for.
The title, description, and slug of an exhibit are all directly accessible from the Exhibit object or the exhibits table, and the tags are accessible from the Exhibit object as well (though they're stored in a different table).
Any library or code already exhibit in Omeka project can I use when I am building this feature.
Zend Framework (which Omeka is built on) has a few libraries for creating XML or RSS feeds. Zend_Feed_Writer is probably the best of them.
As for getting the exhibit data, the ExhibitTable and Exhibit models should be all you'd need.