The developers working for Sony Mobile surprised us all when they announced a very much functioning build of Android 4.3 Jelly Bean AOSP (Android Open Source Project) ROM for the Xperia Tablet Z. Well, it's time for the Xperia Z and ZL to receive the same treatment.
Alternatively, you can download the Android 4.3 AOSP ROM for the Xperia Z from the XDA-Developers Forum thread, maintained by developer pulser_g2.
Mind you, to flash the ROM you need to first unlock your phone's bootloader (the method can be found on the Sony Mobile website). Also note that these ROMs are not complete, in the sense that many of the features are broken or are yet to be implemented and hence, Sony Mobile marks these ROMs as "not intended for daily use".