All the answers are already out there, you just need to use the search button:
- How-to: Standalones and HVMs | Qubes OS
- Troubleshooting: HVM troubleshooting | Qubes OS
- Tips: Contents/docs/os/linux-hvm-tips.md at master · Qubes-Community/Contents · GitHub
- More troubleshooting: Re: [qubes-users] win7 HVM will not start with RAM > 3GB with "Failed to load VM config" - #3 by marmarek
It would help to know, exactly which of the above you’ve tried.