The steps for adding XML Site map in a website:- 1)First google a site map generator and you will find one like xml-sitemaps.com 2)In that give your website address and generate one xml site map, download it to your pc. 3)Login to your server of the website and upload the XML sitemap file to the server where the website is hosted. Now its done. The HTML Site maps is added by website developers, if you are a SEO then no need to bother about that.
Actualy, HTML sitemaps can unlike XML sitemaps distribute actual link juice around your site. Thus you should not ignore them for SEO. (To learn more about sitemap file kinds, check this article.)