sony published a video showing how to unlock bootloader on xperia devices

bootloader on  is a set of codes that runs before the actual OS runs this is locked by the manufacturers as they want users to use their custom made ROMS and prevent you to flash other ROMS so if you unlock your bootloader it voids your warranty.

sony is known to be very dev friendly and has posted a video on its youtube dev channel which shows the procedure for unlocking bootloader of xperia devices. they state that the procedure can void warranty and is not recommended for standard users



to unlock your bootloader go here - http://developer.sonymobile.com/unlockbootloader/
select your device
download SDK
and fastboot drivers
intall all USB drivers by running SDK manager
copy fastboot file to USB drivers
enable USB debugging
turn off your phone and connect it via USB
and unlock it via fast boot