For　decades,　network　virtualization　plays　a　crucial　role　in　modern　networks,　e.g.,　6G　mobile　networks:　tenants　construct　arbitrary　virtual　networks　on　the　same　physical　network.　However,　production　networks　are　becoming　programmable　and　need　more　ways　of　network　configurations.　So　prior　techniques　like　SDN　hypervisors　are　deprecated　since　they　cannot　configure　programmable　networks.　In　response,　a　new　category　of　network　virtualization　techniques　has　emerged　recently　to　virtualize　programmable　networks,　i.e.,　programmable　network　hypervisors　(PNHs).　This　article　first　reviews　this　trend　to　comprehensively　survey　PNHs.　To　understand　their　benefits,　we　build　a　testbed　with　hardware　switches　and　sophisticated　topologies.　Our　testbed　experiments　stress-test　hypervisors　and　demonstrate　that　they　outperform　prior　works　with　higher　acceptance　ratios　of　virtual　networks　under　large-scale　scenarios　and　heavy　loads.　Finally,　we　highlight　the　open　issues　of　these　techniques　to　shed　light　on　the　future　directions　in　this　promising　topic.　©　1986-2012　IEEE.