XML Sitemap: An XML file that is available to web crawlers (Google Bot) only; and not for the user(humans). The XML file has to be uploaded in the root directory of your website. It determines the crawler, that how many pages presented on your website, recent modifications in webpages and frequency of modification and timeline.

HTML Sitemap: This is available for the user and crawler. Any user can see an HTML site map of the website. Normally big e-commerce site like Amazon, or Flipkart has HTML sitemap. since it is easily accessible by humans so does not tell the last modification of webpages and frequency of modification and timeline to humans.