Strapi is a headless CMS - meaning there is no presentation layer where you can view your content. You just get a JSON back. Webiny comes with a visual page builder, as well as a presentation layer. At this moment Webiny doesn't have a headless module - but that's something we are working on.

Keystone is more a web framework with a base set of modules, so I would say it's similar to Webiny. However Webiny is designed and built from ground up to run inside a serverless environment.