Android 14 was released recently and this is a test GSI image of Android 14 based on AOSP made by developer AndyYan.
This file is in .xz format so you need to extract this before you can use it.
You need the xz-utils package to be able to extract this file.
sudo apt-get install xz-utils or yum install xz-utils
Once you have installed the tool, you can use any of the command below to extract the file:
unxz system-14-bvS.img.xz or xz --decompress system-14-bvS.img.xz
After extracting you will get the system-14-bvS.img file which can be flashed through fastboot.