Some kernel/Xorg combination use only 640x480 in HVM, which is quite small. To enable maximum resolution, some changes in Xorg configuration are needed:
1. Force "vesa" video driver
2. Provide wide horizontal synchronization range
To achieve it (all commands run as root):
1. Generate XOrg configuratio (if you don't have it):
Now you should get at least 1280x1024 and be able to choose other modes.
Qubes agents
------------
Linux Qubes agents are written with PV domain in mind, but it looks to be possible to run them also in HVM domain. However some work is required to achieve it. Check [this thread](https://groups.google.com/group/qubes-devel/browse_thread/thread/081df4a43e49e7a5).