Arvo is a purely functional, non-preemptive OS, written in Hoon, that serves as the event manager of your urbit. It can upgrade itself from over the network without downtime. The Arvo kernel proper is quite simple -- it's only about 600 lines of code, excluding its various modules.

The Urbit transition function is implemented in Arvo. Upon being 'poked' by Vere with the pair of <input event, state>, Arvo directs the event to the appropriate OS module. The result of each Vere 'poke' is a pair of <output events, new state>. Events are typed, and each has an explicit call-stack structure indicating the event's source module in Arvo.

For a more in-depth technical introduction, see Arvo Overview.

Arvo modules are also called 'vanes'. Arvo's vanes are:

Ames: defines and implements Urbit's encrypted P2P network protocol, as well as Urbit's identity protocol.
Behn: manages timer events for other vanes.
Clay: global, version-controlled, and referentially-transparent file system. Also includes our typed functional build system.
Dill: terminal driver.
Eyre: HTTP server.
Gall: application sandbox and manager.
Iris: HTTP client.
Jael: Public and private key storage.
Khan: Control plane and thread runner.