LFO is a type of VCO a Low Frequency Oscillator that can be one of various wave shapes like a square or pulse wave form. Envelopes are not oscillators. In other words, they are fixed shape but that can vary with different stages: attack, sustain, decay and release. Here is a good comparison:
https://www.musicradar.com/how-to/LFOs-and-modulation-for-beginners
Now where LFOs and envelopes share they are both modulation sources that can affect parameters of an oscillator. For example, if you want to affect the duration of a note, both can vary that to some degree.
The difference between modulating a parameter with an LFO vs. an Envelope is that LFO’s are intended to continuously repeat at a certain rate or frequency, whereas envelopes are usually triggered events based on some user input like a key press. The envelope generates data based on the duration of the event.
Now the fine line gets blurred in function generator modules like Maths and Quadrax that combine LFOs with envelopes. These super modulators let you configure LFO and envelopes in combos. Acid Rain Labs Maestro is a super duper modulator that I recently obtained which lets you mix wave shapes, LFOs and envelopes in six channels. Quadrax can do similar things.
Hope this helps!