xml file is for search engine spider to see all the url within your site while html file is for search engine to easily locate the structure of your site and also to indexed keywords within the html sitemap.
My largest blog is not using a sitemap and new pages are typically indexed within an hour. I have seen some posts in a #1 position in Google within an hour.
Google may be tied into pingomatic. That could be how they are picking up the new posts.
I have a problem with my sitemap....I found out in webmaster tools that there is a sitemap error due to Dns issue...Does anyone know how to correct it...The site is working fine...and the sitemap is loading fine as well!!
What is the exact error message from Google Webmaster Tools?
If it's a DNS issue, then there probably is nothing wrong with your sitemap.
That error is saying that Google is having trouble resolving the name of your web site into an IP address.
What is the URL of the web site in question?