It's essentially a little bit of industry jargon. Flash is non-volatile, and can be available via an NVMe interface. NVM in this context references essentially

It's essentially a little bit of industry jargon. Flash is non-volatile, and can be available via an NVMe interface. NVM in this context references essentially, some faster-than-Flash non-volatile memory. Since there are multiple tech contenders for this class of NVM, papers often leave it unspecified; but it can range from battery/supercap backed DRAM to PCM, to whatever Intel is using for Optane, or STT-MRAM, or others. Even more confusingly, NVM can be available over NVMe as block access or memory bus interfaces as "byte accessible", or even byte accessible on a memory bus transformed into block access software interfaces, and then there are other PCI level interfaces too. So you can think of NVM in this paper as something like at least an order of magnitude faster than flash access.