These three types usually confuse novice React developers. It seems like they are the same thing, just named differently.
But it's not quite right.

JSX.Element vs ReactElement

Both types are the result of React.createElement()/jsx() function call.

They are both objects with:

• type
• props
• key
• a couple of other "hidden" properties, like ref, $$typeof, etc

ReactElement

ReactElement type is the most basic of all. It's even defined in React source code using flow!

// ./packages/shared/ReactElementType.js

export type ReactElement = {|
  $$typeof: any,
  type: any,
  key: any,
  ref: any,
  props: any,
  // ReactFiber
  _owner: any,

  // __DEV__
  _store: {validated: boolean, ...},
  _self: React$Element<any>,
  _shadowChildren: any,
  _source: Source,
|};

This type is also defined in DefinitelyTyped package.

interface ReactElement<P = any, T extends string | JSXElementConstructor<any> = string | JSXElementConstructor<any>> {
  type: T;
  props: P;
  key: Key | null;
}

JSX.Element

It's more generic type. The key difference is that props and type are typed as any in JSX.Element.

declare global {
  namespace JSX {
    interface Element extends React.ReactElement<any, any> { }
    // ...
  }
}   

This gives flexibility in how different libraries implement JSX.
For example, Preact has its own implementation with different API.

ReactNode

ReactNode type is a different thing. It's not a return value of React.createElement()/jsx() function call.

const Component = () => {
  // Here it's ReactElement
  return <div>Hello world!</div>
}

// Here it's ReactNode
const Example = Component();

React node itself is a representation of the virtual DOM. So ReactNode is the set of all possible return values of a component.

type ReactChild = ReactElement | ReactText;

type ReactFragment = {} | Iterable<ReactNode>;

interface ReactPortal extends ReactElement {
  key: Key | null;
  children: ReactNode;
}

type ReactNode =
  | ReactChild
  | ReactFragment
  | ReactPortal
  | boolean
  | null
  | undefined;

What to use for children?

Generally speaking, ReactNode is the correct way to type the children prop. It gives the most flexibility while maintaining the proper type checking.

But it has a caveat, because ReactFragment allows a {} type.

const Item = ({ children }: { children: ReactNode }) => {
  return <li>{children}</li>;
}

const App = () => {
  return (
    <ul>
      // Run-time error here, objects are not valid children!
      <Item>{{}}</Item>
    </ul>
  );
}

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