Its an exciting day for the Nexus users as Google has announced on their twitter that beginning today Nexus 7 and Nexus 10 with WiFi only users will start receiving Android 4.4 KitKat updates on their devices.
Starting today, Nexus 7 (2012 and 2013) and Nexus 10 will be getting a tasty update to Android 4.4, KitKat
— Android (@Android) November 13, 2013
The updates are not yet available for the cellular Nexus devices including the Nexus 7 cellular and the Nexus 4 and Google has indicated that they will be made available very soon. Stay tuned for further updates on the same.
You can view the changelog for Android 4.4 KitKat at this link.