paravirtual

Paravirtualization is a virtualization approach in which the guest operating system is modified to run directly on a hypervisor. Unlike traditional full virtualization, where the guest is unaware of the virtualization layer and hardware instructions are emulated, paravirtualization replaces or augments sensitive operations with explicit hypervisor calls. This cooperation between guest and hypervisor can reduce overhead and improve performance.

In a paravirtualized setup, the guest kernel is aware it is running under virtualization and uses a set of hypercalls or a para-virtualized interface for privileged actions such as memory management, scheduling, and I/O. The hypervisor handles these requests more efficiently than hardware emulation would, often avoiding costly trapping and translation. I/O, including network and disk operations, is commonly implemented via para-virtualized devices and drivers that communicate with the hypervisor through a standardized interface, such as virtio.

Advantages of paravirtualization include lower overhead, higher throughput, and better performance for certain workloads, especially on hardware without advanced virtualization extensions. It can also enable tighter resource sharing and predictable performance in multi-tenant environments. However, the approach requires the guest operating system to be modified to support the paravirtualized interface, which reduces portability and increases maintenance when compared with unmodified guests.

Historically, paravirtualization was central to Xen's PV mode and influenced the development of para-virtualized I/O like virtio. As hardware-assisted virtualization and full virtualization matured, the share of PV-based deployments decreased, though PV concepts remain relevant in specific hypervisor configurations and embedded or high-performance virtualization scenarios.