What OS kernels are out there?
Well, for starters, there’s the big 3: Darwin, Linux, and Windows NT.
Darwin is the kernel behind all of Apple’s operating systems. These include iOS, macOS, watchOS, and tvOS. Darwin itself is based off of NeXTSTEP, which was based off of BSD Unix. Darwin is mostly open-source, however certain components remain closed-source. Darwin is developed and maintained mainly by Apple.
Linux is the kernel behind a multitude of operating systems, including GNU/Linux, Android, Tizen, ChromeOS, as well as other custom operating systems designed to run on self-driving cars, smart TVs, and the ISS (International Space Station). Linux was originally made as a Minix clone, but has evolved a lot since then. Linux is fully open-source. Linux is developed and maintained mainly by its namesake, Linus Torvalds.
Windows NT is the kernel behind the world’s most popular desktop OS, Windows. It is also behind the (now non-existent) Windows Mobile, as well as Windows IoT (Internet of Things). Windows NT was developed from scratch and is not based off of any other kernel. Windows NT is fully closed-source. Windows NT is developed and maintained by Microsoft.
Then, you have the smaller kernels such as:
*BSD kernels. The BSDs such as FreeBSD, OpenBSD, NetBSD, etc. all have their own kernels tailored for their operating systems. Although they are all quite similar, and share their roots in the original Berkeley UNIX, they deserve a mention here due to each variant of BSD having its own kernel.
GNU Hurd. Originally, when Richard Stallman and his team were developing the GNU operating system, they were also working on their own kernel called Hurd. However, around those days Linus Torvalds unveiled Linux and the GNU team chose Linux as the kernel to go with their operating system. That doesn’t mean that the Hurd is dead, however. It is still in beta, and is being kept alive mostly as an interesting technical project. It is not used at all in production use, and is mostly used by hobbyists who like to tinker.
Minix. Minix is a Unix clone that was used mostly for educational purposes. When Linus Torvalds was studying computer science, he got so frustrated of Minix that he decided to write his own kernel, and that was how Linux came into existence. Minix is still alive these days, however it is rarely used.
And many more. In fact, the above list is just a drop in the ocean of operating system kernels.
If you want more information on this subject, I suggest you check out the link in the footnotes.
 Comparison of operating system kernels – Wikipedia
Link to Quora: