no such file to load -- xml upgrading rails2 to rails3

I am sucked about the below issues which I am facing on upgrading application from rails2 to rails3...

no such file to load -- xml

rake/rdoctask is deprecated. Use rdoc/task instead (in RDoc 2.4.2+) rubies/ruby-1.9.2-p330/lib/ruby/1.9.1/rdoc/task.rb:30: warning: already initialized constant Task rake aborted! no such file to load -- xml rubies/ruby-1.9.2-p330/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'