Technology/VMs, (sdf.org), 02/19/2019
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I just read the oldest reference to "virtual machines" that 
I know of, dating back to a technical report for DARPA in 
1975[1]. Of course, I know very little about the history of 
virtual machines and/or emulation, and I came across the 
reference by accident while searching for other information.
I know enough to realize that the term "virtual machine" has 
come to have a particular definition that may not match the 
one used here; still, it was interesting to read, and so 
I've copied the reference paragraph below (from page 14 of 
the document):

"The PRIM (Programming Research Instrument) project has 
completed an interactive microprogramming facility to be 
used as a general-purpose emulation laboratory for creating, 
manipulating, and debugging arbitrary computer architectures 
and high-level language processors. A unique service on the 
ARPANET, it consists of a powerful sharable micro-
programmable CPU (the MLP-900) closely coupled with a TENEX 
system and appropriate software to allow interactive access 
to, and control of, the computing environment the user 
wishes to emulate. The MLP-900 has proved to be reliable 
in continuous operation since August 1974, with the primary 
appications being the emulation of existing minicomputers 
for experimentation and evaluation. A library of emulators 
is being developed as user population grows; the library 
currently consists of a basic PDP-10 (developed as a test 
vehicle for the system), a PDP-8, a PDP-11, an AN/UYK-20, a 
Univac 1050 MOD2, and a Nova (CPU only). As the emulator 
library continues to grow, the PRIM facility should become 
more attractive to a larger user community. Current plans 
also include allowing users access to these virtual 
machines (emulators) via a National Software Works Tool 
Bearing Host. This will make available several new NSW tools 
with a single interface. (See Section 3 for a further 
discussion of the National Software Works (NSW).) PRIM is 
therefore becoming a service facility, providing a unique 
tool to groups of NSW programmers, as well as an 
experimental computer environment for the researcher."

[1] https://apps.dtic.mil/dtic/tr/fulltext/u2/a016662.pdf