When I use DEBUG int 10/4F00 to see the info, it says I have 4095*64KB=255.9375MB memory to use, and the windows info also give 260MB address(Exxxxxxx, F7(8-B)xxxxx).
What do the rest of the video memory do? Is it unaccessable?

You can probably use for scrolling. Just change the start offset from the default of zero and now you can see other parts of the memory on the screen.

I guess scrolling uses the 256MB is enough, but my vm is 1GB

I suspect the OS is supposed to have the necessary driver to properly configure the video card for the desired purpose and use all the features. It is not the job of the BIOS to do that. The BIOS gives you enough to get things up, but further than that you'll need to do yourself.

The Ex..x range will be just the VRAM and the Fx..x ranges tend to be expansion ROM areas (there are also the legacy address ranges and possibly others related to ACPI). It is possible to obtain [more] exact information by enumerating the PCI bus.