Xml is the one you submit to the SE's. Html is the one you put on your website for easier navigation if your customers want to see all your pages on one page. The SE's will give give you credit for having a sitemap on your website.
HTML sitemap is used to list all hyperlinks of different sections and pages of your blog/website where XML sitemap lists URLs (world wide web addresses) for your blog/website in a special format
Sitemap.xml is for search engines to know which pages are there in our website? And sitemap.html is for the visitors to know all the pages of your site. It is good for us to have both sitemaps in our site.
Sitemap in XML is just like Your pondicherry times for Robots to crawl, while Sitemap in HTML is for the Readers or Visitors, so it's giving them an Easier Navigation. For instance: My Blog Sitemap.