When I try to submit the sitemap to google.com/webmastertools it shows a red cross in the status. This is probably because I have to submit the url of sitemap as: www.domainname.com/sitemap.xml But I cannot download the sitemap through this url. It can be downloaded from domainname.com/sitemap.xml (without www). I am not being able to figure out the problem. What should be done? Please help
Did you submit the URL correctly? I believe the www should be no problem -- if you use sub-domain there is no www and it works for sub-domains.
Did you already checked the way you submitted your website there? with http:// / without? Big difference...
Have you got your website being verified already? If not, please do it. You cannot submit sitemap via Google Webmaster Tools unless you have your website verified first.