Sony has officially announced that the Android M (Developer Preview) will be made available on selected devices. They have also listed the devices that will get access to Android M. But do note that this is a Developer preview, it is not stable release.
The following devices will get access to the Android M Developer Preview (DP) – Xperia Z3/Z3 Compact/Z3 tablet, Z2/Z2 Tablet, Z1/Z1 compact, Z Ultra, E3, M2, T3 and T2 Ultra. Now this software is not certified and not meant for regular usage. If you know what you are doing you can test the Android M much before it is officially made available. But do note that some features have been disabled.
Sony has specifically mentioned that camera and Modem are disabled but can be installed later on. This is not an OTA update, but you will need to use the test image and manually flash it on to your device.
For more info visit SONY Developer site.