A unikernel could be designed as an RTOS, but not all RTOS are unikernels (in fact, I'm not aware that any are). Indeed, QNX, a RTOS, is not implemented using a single address space, nor is your application linked against the kernel and supporting libraries into a unikernel. While it's certainly possible that you could implement an application/unikernel with real-time guarantees, I'm not aware of a specific unikernel implementation that targets that space specifically.

Thanks or your kind reply, I use to find difficult to classify between RTOS and unikernels technically.

A RTOS means a specific thing regarding timing (and sometimes fairness) guarantees. A unikernel isn't that.