VCO, also called Voltage Controlled Oscillaor, is a combination of components (like resistors, capacitors and transistors) to oscillator and store RF energy in order to create a certain frequency by changing the voltage. VCXO, also called Voltage Controlled X-tal (crystal) Oscillator, is a x-tal oscillator controlled by a voltage. A x-tal is cheap and temperature sensitive element which must be controlled by a voltage for frequency stability within a few percent in variation, that is sufficient for applications such as cellular phones. TCXO, also called Temperature Controlled X-tal (crystal) Oscillator, is a temperature controlled X-tal oscillator with a reduced temperature  dependence of the resonance frequency. By using a temperature sensor controlling the tuning voltage stability in the ppm range is attained.