How free is Mavericks? How free is Mavericks?
Is it free as a free beer?

I want to run in on virtual machine and do not own licence for it (but it's free, right?) - is it OK?

What about hard to find solutions of newest OS - easiest thing to do is to install older system like Snow Leopard and upgrade to Mavericks. Snow Leopard would be at that point without licence, but solely in purpose for installing free new system.

How does Apple licencing define that "free" Mavericks?
You can download the OS X Mavericks license agreement from Apple's Software License Agreements page. It's definitively not free of either license or copyright. The licensing terms stipulate only running it on Apple hardware and furthermore restricted on how to use it with virtual machines on top of Apple hardware. You need to also consider the App Store license terms for your country if you download the free-of-cost Mavericks software from that store.
Below is a relevant extract from the OS X Mavericks licence:

2. Permitted License Uses and Restrictions.
A. Preinstalled and Single-Copy Apple Software License. Subject to the terms and conditions of this License, unless you obtained the Apple Software from the Mac App Store or under a volume license, maintenance or other written agreement from Apple, you are granted a limited, non- exclusive license to install, use and run one (1) copy of the Apple Software on a single Apple- branded computer at any one time. For example, these single-copy license terms apply to you if you obtained the Apple Software preinstalled on Apple-branded hardware.
B. Mac App Store License. If you obtained a license for the Apple Software from the Mac App Store, then subject to the terms and conditions of this License and as permitted by the Mac App Store Usage Rules set forth in the App Store Terms and Conditions (http://www.apple.com/legal/itunes/ww/) (“Usage Rules”), you are granted a limited, non-transferable, non-exclusive license:
(i) to download, install, use and run for personal, non-commercial use, one (1) copy of the Apple Software directly on each Apple-branded computer running OS X Mountain Lion, OS X Lion or OS X Snow Leopard (“Mac Computer”) that you own or control;
(ii) If you are a commercial enterprise or educational institution, to download, install, use and run one (1) copy of the Apple Software for use either: (a) by a single individual on each of the Mac Computer(s) that you own or control, or (b) by multiple individuals on a single shared Mac Computer that you own or control. For example, a single employee may use the Apple Software on both the employee’s desktop Mac Computer and laptop Mac Computer, or multiple students may serially use the Apple Software on a single Mac Computer located at a resource center or library; and
(iii) to install, use and run up to two (2) additional copies or instances of the Apple Software within virtual operating system environments on each Mac Computer you own or control that is already running the Apple Software, for purposes of: (a) software development; (b) testing during software development; (c) using OS X Server; or (d) personal, non-commercial use.
The grant set forth in Section 2B(iii) above does not permit you to use the virtualized copies or instances of the Apple Software in connection with service bureau, time-sharing, terminal sharing or other similar types of services.

Bold added for highlighting purposes; the bolding is not present in the original licence.