How to generate an XML feed?
You can do it using the Views Data Export module (Most popular)


  This module is designed to provide a way to export large amounts of
  data from views. It provides a display plugin that can rendered
  progressively in a batch. Style plugins are included that support
  exporting in the following types:
  
  
  CSV
  Microsoft XLS (or really simple HTML understood by Excel)
  Microsoft DOC
  Basic TXT
  XML
  


Example:



Alternatively, you can also do it using the Views Datasource module


  Views Datasource is a set of plugins for Drupal Views for rendering
  content in a number of shareable, reusable formats based on XML, JSON
  and XHTML. These formats allow content in a Drupal site to be easily
  used as data sources for Semantic Web clients and web mash-ups. Views
  Datasource plugins output content from node lists created in the
  Drupal Views interface in a variety of formats - XML data documents
  using schemas like OPML and Atom, RDF data documents using a
  vocabulary like FOAF, JSON data documents in a format like Exhibit
  JSON, and XHTML data documents using a microformat like hCard.