Light emission from carbon dots (CDs) is of great interest in both electroluminescence and photoluminescence. Herein, we construct electroluminescent and photoluminescent light-emitting diodes (LEDs) from emitters of CDs. The electroluminescent LED with host-guest-doped dual emissive layers (EMLs) of [poly(9-vinylcarbazole) (PVK)-CDs] × 2 gives satisfactory electro-optical properties, with maximum luminance of 560 cd m-2 at 16 V, luminous efficiency of 0.183 cd A-1, power efficiency of 0.082 lm W-1, and external quantum efficiency of 0.25%, which are superior to the counterparts with single-EML of CDs, single-EML of [PVK-CDs], and triple-EMLs of [PVK-CDs] × 3. These enhanced properties are rationally ascribed to optimization of the device architecture, carrier balance improvement, and reduction in concentration-induced quenching. The electroluminescent LEDs also show color evolution from PVK to CDs, and/or to the PVK/CDs interface, with increasing driving voltages, owing to incomplete energy transfer from PVK to CDs. Highly efficient photoluminescent LEDs with 365- and 395-nm UV excitation are demonstrated. With a CDs : polyvinyl pyrrolidone ratio of 1 : 3, the 365-nm excited photoluminescent LED gives a maximum luminance of 512 347 cd m-2 with a power efficiency of 25.2 lm W-1, while the 395-nm excited photoluminescent device gives a maximum luminance of 670 954 cd m-2 with a power efficiency of 22.0 lm W-1, with both showing yellow emission. Our experiments provide some new ideas for broadening CD applications and advancing LEDs.