Kivy will do desktop and mobile, but web applications are a different animal completely. Since they run on a web browser, rather than directly on the computer operating system, your ability to interact with the user's environment is very limited. Anything complex must be written in JavaScript. There are a few Python-to-JavaScript "transpiler" projects out there which attempt to make it possible to write python code to run in a browser, but I have never heard of kivy running on one of them. I think your best bet would be to either plan on doing two implementations, or have your website be a portal to download your desktop app.