As both XML- and HTML-sitemaps are used by search engines to find, and hopefully index, new content faster, you should update all used Sitemap types once new content and URLs are available.
How to (automatically) update sitemaps
If you have a small website where content changes happen either on a weekly- or monthly-basis, you can simply create an XML- and HTML-sitemap by hand, any time new content is available and upload it to your webspace.
If you have a website with many pages and an almost daily update frequency, such as a blog, it is quite handy if you can automatically generate a new sitemap anytime new content is ready.
Many of the different Content Management Systems (CMS) that are available have their own add-ons (plugins) that can automatically create sitemaps for you. The one thing to keep in mind with these plugins is that each additional CMS plugin will lead to a (minimal) worsening of your website’s loading time (PageSpeed).
A pay-to-use alternative for an automatic sitemap creation, which is not bound to any CMS, is being offered, among others, by XML-Sitemaps.com as a SaaS- (Software as a Service) or self-hosted-solution:
- SaaS: https://pro-sitemaps.com/ (free for up to 500 pages)
- Self-hosted: http://www.xml-sitemaps.com/standalone-google-sitemap-generator.html
More alternatives automatically generating sitemaps: