Can’t Install Windows Server 2025 on Hyper-V 2019? Check CPU Compatibility Mode
The customer runs on the 3 node Windows Server 2019 failover cluster. We trying to install the latest Windows Server 2025 as guest VM, but VM not booting and failing with CPU compatibility enabled. Why i have CPU compatibility enabled – because a mix of the CPU models in the cluster – we need the live-migration.
When CPU Compatibility Mode is enabled, a Windows Server 2025 virtual machine (VM) may not run stably on a Hyper-V 2019 cluster due to CPU instruction set limitations. However, if this mode is disabled, the VM might run normally if the hardware supports it.
The issue isn’t that Windows Server 2025 won’t work on a 2019 cluster, but rather a problem with CPU Compatibility Mode on Hyper-V 2019 does not support the new instructions in Windows Server 2025
- CPU Compatibility Mode in Hyper-V 2019 limits the CPU instruction set to a baseline level to ensure the VM can live-migrate between hosts with different CPUs.
- Windows Server 2025 requires newer CPU instructions or features (e.g., AVX512, VBS, MBEC, etc.), which may not be supported or are „hidden“ when compatibility mode is enabled on an older host like 2019.
- In contrast, Hyper-V 2022 supports a higher level of CPU compatibility, so it does not encounter this error.
Therefore, you should:
- Avoid enabling CPU Compatibility Mode for Windows Server 2025 VMs on Hyper-V 2019.
- If you need live-migration functionality between cluster nodes, upgrade your Hyper-V cluster to Windows Server 2022 or newer.
- Check the Event Log for CPU-related errors or VM Worker Process failures when the VM crashes to confirm the exact cause.When CPU Compatibility Mode is enabled, a Windows Server 2025 virtual machine (VM) may not run stably on a Hyper-V 2019 cluster due to CPU instruction set limitations. However, if this mode is disabled, the VM might run normally if the hardware supports it.