Beyond disk formats, the virtual hardware signature differs. A VM built for ESXi expects the VMware SVGA II graphics adapter, the VMXNET3 network driver, and the LSI Logic SAS storage controller. Boot that same disk image on Hyper-V, which presents a Synthetic Network Adapter and a Hyper-V SCSI controller, and the guest OS will crash with a blue screen (INACCESSIBLE_BOOT_DEVICE). This is the core problem: a VM is bound to its hypervisor’s driver ecosystem. The VMConverter’s primary task is to transcend these incompatibilities by manipulating both the disk geometry and the OS configuration. Modern VMConverters (such as VMware vCenter Converter, StarWind V2V Converter, Microsoft Virtual Machine Converter, and open-source tools like qemu-img with virt-v2v ) operate through one of two fundamental paradigms: cold migration (offline conversion) or hot migration (live conversion).
The first wave of cloud migration relied on “lift and shift”—taking on-premise VMs and converting them to cloud-native instances. AWS VM Import/Export, Azure Site Recovery, and Google Cloud Migrate all embed VMConverter logic. They convert VMDK/VHD to AMI or managed disk formats, reconfiguring the bootloader for cloud-init and replacing the kernel for cloud-optimized drivers. Without these converters, the hybrid cloud would be a patchwork of incompatible silos. vmconverter
The source VM remains running while conversion occurs. This is far more complex. The converter installs an agent (or uses a hypervisor’s native API) to take a point-in-time snapshot. It then reads the snapshot’s blocks, converts them, and writes to the target. Meanwhile, it tracks all new writes to the source disk (the “dirty block log”). Once the initial copy is complete, the converter pauses the source VM briefly, syncs the dirty blocks, transfers control, and boots the target VM. VMware vCenter Converter’s “hot cloning” is a classic example. This minimizes downtime to seconds but risks data inconsistency if the dirty block tracking fails. Beyond disk formats, the virtual hardware signature differs
VMware’s .vmdk (Virtual Machine Disk) format, for instance, includes a descriptor file and a sparse extent, supporting snapshots and delta disks. Microsoft’s .vhdx format features a 4KB logical sector size, large block allocation, and a metadata region for BAT (Block Allocation Table). KVM’s qcow2 (QEMU Copy-On-Write 2) offers refcount tables, backing files, and advanced compression. A raw .img file is a simple bit-for-bit linear mapping. This is the core problem: a VM is
We are seeing the emergence of —where a management layer (e.g., Red Hat Virtualization, OpenStack) treats all VM formats as ephemeral. The user requests a VM; the orchestrator pulls the source from any format (VMDK, VHDX, raw) and converts it on-the-fly during streaming boot. This eliminates the distinction between “conversion” and “execution.”