They belong to a category of virtualization known as 'container-based virtualization' which implements virtualized perspectives of the system within the host kernel, effectively dividing the system in to multiple systems (from the perspective of the affected processes). That is unquestionably virtualization.

I believe a statement closer to what you were looking to express was containers are not running under a hypervisor.

LXC still creates a performance impact, depending upon which options are selected, with particular note for the various memory accounting options. However, that impact is far lower than a hypervisor. In addition, startup times are vastly reduced since the kernel bootstrap and hardware detection concerns are rendered unnecessary.