Hi, The first is for robots and the second for humans (if I understand your question correctly). The XML sitemap lists all your site page URLs (and other optional information about the page and any image URLs for images on the page, if appropriate). Search engine robots (Google, Yahoo, etc) look for a sitemap to help them discover your pages (depending on how complex your site is they might be able to do this without a sitemap). Usually placed in your root directory and not designed to be accessed by humans, although you can view it if it exists by following http://www.somedomain.com/sitemap.xml. The HTML sitemap is a page that you or I might look at if we can't find what we're looking for in a site using the site's menus. Usually a list of links and descriptions of all the site's pages.
XML (eXtensible Markup Language) was designed to transport and store data, with focus on what data is and HTML(Hyper Text Markup Language) was designed to display data, with focus on how data looks. XML site maps organize the sections of a web page through URLs using a special format, while the HTML site map is used to sort of organize the different sections and pages of your blog or website through hyperlinks.
HTML Sitemap is for website visitors so that they can find all pages on that website if the site is large. XML Sitemap is for search engine spiders for better indexing of web pages. XML sitemap is more important for website indexing.
XML site map is basically for search engines which will help in improving the crawling rate and the html sitemap is a normal site page which connect via all sites link.