MIUI 7 Global beta ROM 6.3.17 is now officially released and this update bring Android 6.0 Marshmallow to various Xiaomi smartphones. The Android 6.0 Marshmallow is upgrade from materialistic Android 5.1 Lollipop and it has host of new improvements including Doze the battery saving app. You can check out for all the Android Marshmallow features over here.
At the Xiaomi MIUI forum, the update change log for this beta build is available and in the same link there are also links to download the ROM file and the fastboot files required in this case.
Currently the MIUI 7 6.3.17 build is available for Mi 3, Mi 4 and Mi Note smartphone which means that they devices can only enjoy the Marshmallow features.
In short spam of time it will be out officially and maybe it's the MIUI 7.1 update that we are looking forward.
Coming to Android version, there is also developer preview of Android N, the upcoming Android version is available. Android N will get an official name during the upcoming Google I/O event which will be held in May or June 2016.