Could not change resolution of Ubuntu 12.04 desktop higher then 1024x768 while running as Xen DomU

I installed Ubuntu 12.04 server beta (x64) and Xen hypervisor over it (from the ubuntu packages). When I run Ubuntu 10.04 desktop as HVM guest I could not change maximal resolution more then 800x600.

According to xen documentation there is a settings in virtual machine cfg file (named stdvga), which allows to increase resolution of DomU. But when I enable it, maximal resolution drops to 640x480.

Also I tried to get it for Ubuntu 12.04 PVHVM DomU. In this case I could manage maximal resolution of screen to 1024x768.

lcpci shows, that virtual videocard has enough memory (16M) for higher resolutions, but Ubuntu does not allow to change it. (In all cases I tried to change resolution from desktop)

Ubuntu 10.04 HVM DomU. (First line are settings in xen virual machine configuration file)

stdvga=1, videoram=8
lspci -s 00:02.0 -vvv

00:02.0 VGA compatible controller: Technical Corp. Device 1111
        Subsystem: XenSource, Inc. Device 0001
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr-
Stepping- SERR- FastB2B- DisINTx-
        Status: Cap- 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort-
<MAbort- >SERR- <PERR- INTx-
        Latency: 0
        Region 0: Memory at f1000000 (32-bit, prefetchable) [size=8M]

stdvga=1, videoram=16
lspci -s 00:02.0 -vvv

00:02.0 VGA compatible controller: Technical Corp. Device 1111
        Subsystem: XenSource, Inc. Device 0001
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr-
Stepping- SERR- FastB2B- DisINTx-
        Status: Cap- 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort-
<MAbort- >SERR- <PERR- INTx-
        Latency: 0
        Region 0: Memory at f0000000 (32-bit, prefetchable) [size=16M]

stdvga=1, videoram=32
lspci -s 00:02.0 -vvv

00:02.0 VGA compatible controller: Technical Corp. Device 1111
        Subsystem: XenSource, Inc. Device 0001
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr-
Stepping- SERR- FastB2B- DisINTx-
        Status: Cap- 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort-
<MAbort- >SERR- <PERR- INTx-
        Latency: 0
        Region 0: Memory at f0000000 (32-bit, prefetchable) [size=16M]

stdvga=1, videoram=64
lspci -s 00:02.0 -vvv

00:02.0 VGA compatible controller: Technical Corp. Device 1111
        Subsystem: XenSource, Inc. Device 0001
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr-
Stepping- SERR- FastB2B- DisINTx-
        Status: Cap- 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort-
<MAbort- >SERR- <PERR- INTx-
        Latency: 0
        Region 0: Memory at f0000000 (32-bit, prefetchable) [size=16M]

Ubuntu 12.04 PVHVM DomU

stdvga=1, videoram=32
lspci -s 00:02.0 -vvv

00:02.0 VGA compatible controller: Device 1234:1111 (prog-if 00 [VGA controller])
 Subsystem: XenSource, Inc. Device 0001
 Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
 Status: Cap- 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
 Latency: 0
 Region 0: Memory at f0000000 (32-bit, prefetchable) [size=16M]
 Expansion ROM at <unassigned> [disabled]

-----Xen Config for Ubuntu 12.04 desktop PVHVM DomU-----
#
# Kernel + memory size
#
arch='32bit'
kernel='/usr/lib/xen-4.1/boot/hvmloader'
builder='hvm'
#bootloader = '/usr/lib/xen-default/bin/pygrub'

vcpus = '3'
memory = '2048'
shadow_memory = '16'
pae='1'
acpi='1'
apic='1'

#
# Disk device(s).
#
#root = '/dev/xvda2 ro'
disk = [
                  'phy:/dev/ubuntu-server/ubuntu-1204d-disk,xvda,w',
                  'phy:/dev/ubuntu-server/ubuntu-1204d-swap,xvdb,w',
                  'file:/home/xplicit/precise-desktop-i386.iso,xvdc:cdrom,r',
              ]

#device_model = '/usr/lib/xen-4.1/bin/qemu-dm'
boot='cd'
#
# Physical volumes
#

#
# Hostname
#
name = 'ubuntu-1204-desktop'

#
# Networking
#
#dhcp = 'dhcp'
vif = [ 'mac=00:12:04:50:e1:8c,bridge=br0' ]
xen_platform_pci='1'

#gfx_passthru=1
#pci=['00:02.0']

#VNC
sdl='0'
vnc='1'
vnclisten='0.0.0.0'
vncconsole='1'
vncpasswd='.......'
stdvga='1'
videoram='16'
serial='pty'
usbdevice='tablet'
tsc_mode='0'

#
# Behaviour
#
#on_poweroff = 'destroy'
on_reboot = 'restart'
on_crash = 'restart'

vfb=['type=vnc,vncdisplay=10,vncpasswd='.....']

-----End of Xen Config-----

-----Xen Config for Ubuntu 10.04 desktop HVM DomU-----
#
# Kernel + memory size
#
arch='32bit'
kernel='/usr/lib/xen-4.1/boot/hvmloader'
builder='hvm'
#bootloader = '/usr/lib/xen-default/bin/pygrub'

vcpus = '3'
memory = '2048'
shadow_memory = '16'

#
# Disk device(s).
#
#root = '/dev/xvda2 ro'
disk = [
                  'phy:/dev/ubuntu-server/xeng1,xvda,w',
                  'phy:/dev/ubuntu-server/xeng1_swap,xvdb,w',
                  'file:/home/xplicit/ubuntu-10.04-desktop.iso,xvdc:cdrom,r',
              ]

#device_model = '/usr/lib/xen-4.1/bin/qemu-dm'
boot='cd'
#
# Physical volumes
#

#
# Hostname
#
name = 'ubuntu-1004-desktop'

#
# Networking
#
#dhcp = 'dhcp'
vif = [ 'type=ioemu,mac=00:16:3e:50:e1:8c,bridge=br0' ]

#gfx_passthru=1
#pci=['00:02.0']

#VNC
sdl='0'
vnc='1'
vnclisten='0.0.0.0'
vncconsole='1'
vncpasswd='......'
stdvga='0'
videoram='16'
serial='pty'
usbdevice='tablet'

#
# Behaviour
#
on_poweroff = 'destroy'
on_reboot = 'restart'
on_crash = 'restart'

vfb=['type=vnc,vncdisplay=10,vncpasswd='...']
-----End of Xen Config-----