It actually is the same anaconda bug as 694760 in RHEL6.1
(description mostly stolen from there)

Description of problem:
While installing FC15 (i386 or x86_64) as PV DomU over xen hypervisor, the
screen can't be displayed entirely in VNC session. Please refer to the
attachment for the screenshot, this could be inconvenient for users to play
FC15 PV on xen.


Version-Release number of selected component (if applicable):
host : RHEL5.6 (x86-64) with the lastest xen : xen-3.0.3-127.el5 
guest: FC15 (anaconda 15.27, xorg-x11-server 1.10.0-7.fc15)


How reproducible:
100%

Steps to Reproduce:
1. start installation of guest:
 
virt-install -n fc1564inst -l http://download.englab.brq.redhat.com/pub/fedora/linux/development/15/x86_64/os -f /var/lib/xen/images/fc15x86_64nial.img -s 4 --vcpus=1 -r 2048 -p

2. proceed through text installations part till you get to GUI install

  
Actual results:
The the screen of FC15 PV DomU's GUI installer isn't displayed entirely

Expected results:
The screen of FC15 installer should be displayed entirely

Additional info:
It works as expected with FC15 HVM guest, FC13 PV guest, RHEL6.0 PV guest.

xrandr -d :1 -q
Screen 0: minimum 800 x 600, current 800 x 600, maximum 800 x 600
default connected 800x600+0+0 0mm x 0mm
   800x600         0.0*

Bug affects only GUI installer, after installing and rebooting guest's GUI works
as expected.