As the Android 4.4 OTA update began reaching some Nexus devices, Google has released the official Android 4.4 factory images for all Nexus smartphones and tablets (released in the last 18 months). These factory images are now available at Google Developers website.
These images can be used to flash KitKat on your Nexus device but you risk losing all your device data and also make a backup before doing so, you will also need to have unlocked bootloader. Factory images are mostly used when users mess up their pre-loaded firmware while tinkering with it.
Originally announced on October 31, Android 4.4 KitKat brings several improvements and new features to the Android operating system including inbuilt cloud-print support, new keyboard, lock-screen widget improvements, immersive mode, faster multitasking, and better dialler app.
Direct links to download factory images: Nexus 7 2013 (Wi-Fi, 3G/ LTE), Nexus 7 2012 (Wi-Fi, 3G), Nexus 4, Nexus 10. You can also find these links at Google Developers website.