Limbo x86 PC Emulator
Limbo is a qemu-based x86 architecture emulator for android devices. With limbo, you can emulate a complete desktop computer on your device and install various systems (FreeDOS, Windows, KolibriOS, various GNU/Linux distributes and any other x86-compatible OS).
Limbo has a wide selection of settings, allowing you to change available RAM, number of emulated cores, type of CPU, mount hard-drive and cd-rom images, select emulated graphics, network and audio card types and many more.
Access to the emulated desktop is done inside limbo with the inbuilt VNC viewer, SDL mode or an external VNC. To connect to an external VNC, you can use MultiVNC or androidVNC clients available in F-Droid.
On android devices with x86-based CPU (Intel Atom), Limbo can run with KVM mode, allowing for much better perfomance.
Keep in mind, limbo is a complete emulation of another CPU architecture, so you most likely won't be able to run modern operating systems with reasonable perfomance.