Migrating servers from client networks to virtual machines (VMs) on a provider network. A migration appliance is installed or booted on the client network, and a migration initiator is instantiated on the provider network. A VM and associated volumes are instantiated on the provider network. The initiator sends a request for a boot sector to the appliance; the appliance reads the blocks from a volume on the client network, converts the blocks to a format used by the VM, and sends the blocks to the initiator. The initiator boots the VM using the boot sector and the VM begins execution. The initiator then retrieves all data blocks for the VM from volumes on the client network via the appliance, stores the data to the volumes on the provider network, and fulfills requests from the VM from either local volumes or the remote volumes via the appliance.