I think the 3D Xpoint storage works with some phase change material (PCM) to store information. In PCM the crystalline structure can be changed by a current and then remains in that state without any voltage applied.

Geoffrey Taylors patent "Thyristor Memory Cell Integrated Circuit" also uses PCM, but in parallel has a two-thyristor circuitry per memory cell for much greater speed. POET memory can be operated either in slow, non-volatile mode or in very fast, volatile mode. Switching between modes is possible, e.g. by copying the volatile data from the thyristor memory to the non-volatile PCM memory.

While 3D Xpoint's speed is between SSD (fast "harddisk") and DRAM (faster, dedicated memory chips in computers), POET also targets SRAM, which is a ultimately fast cache memory inside the CPU.

3D Xpoint might be cheaper (which remains to be seen) while POET memory has a broader range of possible applications.