How can I copy&paste from the host to a KVM guest?

The solution is simple. Just install the package spice-vdagent in the guest virtual machine:

sudo apt install spice-vdagent

The clipboard is automatically shared - we can copy and paste between the host and the guest.

For my Linux Mint 18.2 Sonya machine timelf123 provided the answer:

• In my Windows guest (Windows 7) downloaded and installed spice-guest-tools-latest.exe.
• Before That, back in my QEMU/KVM Virtual Machine Manager I had to switch the guest machine's display to Spice.
• Before That, back in my Linux host I had to fire up Synaptic Package Manager and install gir1.2-spice-client-gtk-3.0.

In case the question pops up - what if it's Ubuntu server or similar?

Don't run around in circles - install openssh-server on the vm, and when you ssh in you cut and paste into your favourite terminal.

In year 2021, qemu uses qemu-vdagent chardev for copy&paste, which turns off clipboard by default, so simply installing spice-vdagent package in guest OS does not work.

The solution of 2021 is:

1. Install spice-vdagent package as the current answer said,

2. Have qemu support spice (--enable-spice --enable-spice-protocol in ./configure option),

3. Launch qemu using:

   qemu-system-x86_64 [ ... ] \
     -chardev qemu-vdagent,id=ch1,name=vdagent,clipboard=on \
     -device virtio-serial-pci \
     -device virtserialport,chardev=ch1,id=ch1,name=com.redhat.spice.0
   

   P.S. A side effect on my qemu is that the mouse moves more smoothly thanks to vdagent.

Reference: https://www.kraxel.org/blog/2021/05/qemu-cut-paste/